Go to the cheats screen from the main menu and click on keyboard. Then simply type the cheats in. Make sure you uncapitalize the first letter because it automatically capitalizes it for you.

Password	Effect
iamgod	Enemies can't hit you.
idontreload	Infinite ammo and you don't need to reload.
Iwhanttokill	One-shot kills.
iamfast	Run five times faster.
iwhanteverything	Turns on all cheats.
iaminvisible	You become invisible.
iwhantmoney	You get $3000000000.
iwhantitems	You get all the items.
iwhantweapons	You get all the weapons.