While in game press "Enter" and type " " (include the space after ), followed with the one of the codes from below and then press "Enter" to enable the code.
Cheat Code - Effect
mpcanthurtme - God Mode
mpschuckit - Add Weapons & Ammo
mpsmithy - Full Armor
mpkohler - Full Ammo
mpbeamme - Beam Player to Level Start
mpsixthsense - No Clipping
mpicu - Third Person View
mptachometer - Show Speed Info
mpsizeme - Show Size Info
mpstockpile - Full ammunition
mpbunker - Unlimited ammunition
mpxfiles - Level select for all three species
mpdoctordoctor - Set health to indicated number
mpmorph - Change into indicated character type
mpsixthsense - Toggle no clipping
mpgrs - Toggle rotation display
mpgps - Show position info on/off
mpfov - Use keys to edit FOV value
mpvertextint - Use keys to edit vertext tint
mplightadd - Use keys to edit light amplification
mplightscale - Use keys to edit light scale
mpbreach - Use keys to edit weapon breach
mpwmpos - Use keys to edit weapon offset 1
mpwpos - Use keys to edit weapon offset 2
mpmillertime - Marines have slurred voice
mpdeathtoall - Kill everything except your character
Morph
Hit the enter key, then type mpmorph (whatever you want to morph into). For example: mpmorph queen.