You must have a transfer kit to mod a game. Or a JTAG'd Xbox, or a DevKit. I suggest you just buy a Transfer Kit, because you will have no idea what to do with a JTAG, and it is near impossible to get hold of a DevKit. There are tutorials on how to get a free Transfer Kit...I will look around and post back the link in a minutes for you.
[EDIT] Here is the link on how to get a FREE Transfer Kit-working-