During the game, press the " ~ " key, type in the code and press ENTER.

(in some versions press the " ¬ " key, type in the code and press ENTER.)

then in the " ~ or ¬ " console:

iwillsurvive - God mode
raindropskeepfallinonmyhead - Allow call for arty < 5 Shots > can be re-entered for more
takeittothelimit - Ammo reload
hitmewithyourbestshot - Computer rocks
closetoyou - Surprise/ Invisible to the enemy
sky - brighter display
letmego - Level select

Press " T " to bring up a command window

AMMO - you got what I need
REFUEL - big gulp
HEAL - chiliburger