A jailbroken PS3 provides a better modding expierince but im assuming youre not interseted how. So you can download a modded gamsave and put it on a USB flashdrive. When that is all done, plug in your USB to your Playstation and resume the campaign. It should load up then take you to a black screen then redirect you to the main menu. From there you can load up zombies and play with your mod menu (Multiplayer and Singleplayer).