I know modding is harder for mac users. I did some research and found this. I take no credit for this. The source is on the bottom of the page. Enjoy!!
This will be a thread where people will post all of the things that a new Halo for Mac modder will need to know, and all the programs a new person will need.
Programs
HHK: Capable of fast rebuilding, swapping, and more. Not reccommened for brand new users.
HMT: Recommended for new, and advanced users. Can swap offsets, rebuild maps, batch extract maps, use dependancy swapper.
OBJConverter: Use this to import custom models/extract models from Halo.
PPFMaster: Use this to apply and make PPF Patches.
HexEditor: Use this for all your hex editing pleasures.
Wings3D: Use this free modeling program for your modeling pleasures.
Converted Map Patcher: Use this to install the two CE converted maps, Quagmire and NMC.
All of these programs can be found
HERE. Tutorials:
Brief tutorial on how to swap offsets in HMT: By mac_user
Here we will swap the projectile of a tank with the sniper rifle.
1. First, back up your map you want to use.
2. Open the map you want to swap the projectile. Do this by clicking "Open Map" and navigate to your maps folder. It doesn't matter what map you choose, as long as the map has a sniper rifle in it, and its not the UI.map, sounds.map, or bitmaps.map.
3. Once you opened the map, there should be a list of odd looking things, including "bipd". Go down and find the one called "proj" and click the carrot, making a list of all of the projectiles in that certain map.
4. Go down to the tank projectile (should be something similar to vehicles\tank\tank projectile) select it, then click "Swap", which is located on the right side of the window.
5. In the swap window, scroll back down to the "proj" section. It should be highlighted. Open the part and search for the sniper rifle projectile. It should be called something similar to "weapons\sniper rifle\sniper projectile"
6. Click swap.
7. Click "Save Offsets" Located on the right side of the main window.
8. Load up halo and try it out!
Quick Dependancy swapper tutorial by mac_user
In this tutorial we will make the plasma pistol fire out cyborgs.
1. Open a map of your choice.
2. Click in the Window Menu and select Dependancy Swapper.
3. On the left side of the menu, go down to the "weap" section, then look for "weapons\plasma pistol\plasma pistol"
3. When you clicked on the “weapons\plasma pistol\plasma pistol” a list of all the stuff that is in the plasma pistol appears on the right of the dependancy swapping window. On the left side of this section there should be the things that we saw earlier, the “mod2”, “proj” and more. Look for the “proj”. When you found it, click on it. On the bottom of this window there should be a little text box.
4. To make the plasma pistol fire cyborgs, you will need to change the “proj” into a “bipd” (Biped). To do this, type in “bipd” without the quotes into the text box, and hit return.
5. Now, in the list to the right of the text box, there should be two bipds. One of them should be something similar to “cyborg\cyborg” and another called “cyborg\cyborg_mp”.
6. If you wish the cyborg you spawn to be green, choose the “cyborg\cyborg” one, and if you want it to be white, choose “cyborg\cyborg_mp”. Once you have done that, you are done. Close the dependancy swapping window.
7. Open Halo up (You don’t have to save the map or anything) and fire the plasma pistol!
8. If you spawn to many cyborgs, the game can lag seriously if you don’t have a good processor/video card.
9. Experiment with other things. You can put “vehi” in, and have the plasma pistol fire warthogs, and more. You don’t even need to use the plasma pistol either, you can use other weapons.
10. This is an excellent way to spawn banshees in maps that don’t normally have them, such as Hang ‘em High or Battle/Beaver Creek.
Exporting/Importing Models by Totakeke
In this tutorial, we are going to export the Warthog model, edit it, and import it back in Blood Gulch.
1. Open bloodgulch.map in HMT. Scroll down to the mod2 section and look for the vehicles/warthog/warthog. Save the meta nonrecursively onto the desktop.
2. Open OBJConverter and click on the Open button. Choose bloodgulch.map, and then select the meta you saved onto your desktop.
3. Now, make sure that the Geometries tab at the bottom is selected, and select the first geometry in the list in the middle of the window, then select Geometry 1 and then part 0. Click on the “Export to OBJ” button below the Geometries list and save it to your desktop.
4. Now the editing part. I will be using Wings3D as an example. In Wings3D, go to File -> Import -> Wavefront (obj)... and choose the OBJ file you exported with OBJConverter. Now edit it as you please (I will not be explaining how). After you finish, go to File -> Export -> Wavefront (obj), and save it to your desktop.
5. Now switch to OBJConverter and click on the “OBJ” button and select the edited OBJ model file. Save the vertices and indices files on your desktop.
6. Go back to Geometry 1 and part 0. Set all the Verts and Inds to 0 of all the OTHER parts in that geometry [1]. Then click on “Update Offsets”.
7. Now make sure the part 0 of geometry 1 is selected, and click Attach and choose the vertices file and then the indices file you created in step 6. Click “Save” and then save the meta to your desktop.
8. Go to the Misc tab at the bottom of the window and click on the “Append” button. A new window will pop up. Choose the vertices file and then the indices file and click “Append”.
9. Switch to HMT and inject the meta you created in step 8. (it should be called temp.mod2 by default) If it doesn’t let you inject it, rename temp.mod2 to temp.mod2.meta and it should recognize it. HMT should now crash and quit after you click anything, so open it up again, and view the warthog model. It should now look like your edited model.
PPF Creation by bobindashadows
PPFMaster The first thing you need is PPFMaster, a program by ScotiaSoft systems for the creation of PPF Patches. You can download it form the utilities page. Also, you need to have a modded map, along with the original.
A Little Bit About PPFs
The reason we have PPF Patches is that it’s difficult to distribute the full Halo maps, so people can install them. What PPF patches do is figure out what’s different between a regular map and a mod, and save what’s different. That way, a person can simply download your patch, and use a program to install the mod. Note: PPFs are usually not efficient for rebuilt maps. I’m going to teach you how to make those PPFs here.
Making the PPF
First, you have to have both an unmodded map and a modded map. If you don’t have one, stop here. When you have both, launch PPFMaster. There are two sections to the program Ñ apply, and make. You will be looking at the bottom half of the window. Use the two browse buttons to select the original map file, and select the modded one. If you don’t know what file_id.diz is, ignore that. It’s beyond the scope of this tutorial. Choose if you want “Undo data”, which allows the user to undo your mod if they so desire. Choose if you want validation - chances are you don’t. That will just make it harder for people to install your mod. When you’re ready, click the “Make” button. It will ask you where you would like to save your PPF, and proceed to create it. Just wait a while, and it should make the PPF.
Conclusion
For those Mac users wanting to distribute their mods, PPFs are key. This tutorial should’ve taught you how to use a simple program and make the PPFs you need.
Source:
A Starting Point For Newbies - HaloDev Forums 
Mac Users
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