While in gameplay press ~ key to activate the console.

Type "cht_bEnableCheats=1" (without the quotes)
to enable the first set of cheats options such as Max Health, All Guns, Level Selection, kill all enemies.

After entering this command press ~ then ESC and then F1 you should now have the cheat menu. From now on at the bottom of the menu (Load,Save Options,Leaderboards...) you should be able to see Cheats F3, this is the cheats menu.

Type "cht_bEnableCheats=2" (without the quotes)
to enable second set of cheats options God Mode, Ghost, Invisible and Fly mode.

After entering this command press ~ then ESC and then F1 you should now have the cheat menu. From now on at the bottom of the menu (Load,Save Options,Leaderboards...) you should be able to see Cheats F3, this is the cheats menu.