While in game press "Enter" and type [cheat] with [] then a space and 
type the codes given below and then press enter to enable it. 
If you retype it the cheat code gets disabled.

Code            Result 
----------------------
mpcanthurtme  - God mode on/off 
mpschuckit    - Add weapons and ammo 
mpsmithy      - Full armor 
mpkohler      - Full ammo 
mpstockpile   - Full ammo (maybe with differences?) 
mpbeamme      - Beam player to level start with initial conditions 
mpsixthsense  - No clip on/off 
mpicu         - 3rd person mode on/off (HUD doesn't visible in this mode) 
mptachometer  - Show speed info on/off 
mpsizeme      - Show size info on/off 
mpgrs         - Show rotation info on/off 
mpgps         - Show position info on/off 
mpfov         - Edit (with keys) FOV value 
mpvertextint  - Edit (with keys) vertext tint 
mplightadd    - Edit (with keys) light amplification 
mplightscale  - Edit (with keys) light scale 
mpbreach Edit - (with keys) weapon breach 
mpwmpos Edit  - (with keys) weapon offset 1 
mpwpos Edit   - (with keys) weapon offset 2
mpreloadbutes - Reload an attribute file
mptriggers    - Toggle trigger boxes
mpmph         - Displays framerate
mpconfig      - Load new cfg file