Press escape (ESC) during game play and
enter your which ever code you want.
You won't get any confirmation that anything has been enabled.
IFGDGUN - Gives you a pistol.
IFMDAMAGE - (Didn't seem to do much when I tried however
it's supposed to increase damage.)
IFDAURA - Change aura to infernal.
IFUAMMO - Full ammo including grenades and mines.
IFGPLASMA - Gives you a plasma gun.