Enter the Cheat screen code from the main menu to call up the cheat screen. From here you can enter any level and turn on God mode and infinite ammo. Enter the other codes from the main menu to give yourself those abilities -- you will hear a clicking sound to confirm the cheat is active.

Cheat Screen - 24328
Infinite ghost vision - 46266
Infinite clips - 48627
All weapons - 48662
Unlock all single player maps - 67687