go to OPTIONGAME OPTION and ENABLE CONSOLE,
click YES.
In menu title press: ~ and type:
developer 1 = enable cheat codes, LOAD button appears, click it and select the level
Once the level is loaded type ~ again and type "devmap" and you can use the following codes:

god = God Mode
give ammo = All Ammo
give all = All weapons, full ammo, health and armor
noclip = Walk through solid objects
notarget = Invisibility to most enemies
kill = suicide