When you are in gameplay press enter and type in <cheat>mpxfiles and then you will hear a sound of a predator mask that confirms that all the levels are anabled.
In single player mode press 'enter' then type, "<cheat>" then leave a space and enter one of the following codes:
For example to enable god mode, you would type the following:
"<cheat> mpcanthurtme" then press enter again.
mpcanthurtme= toggle god mode
mpschuckit= all weapons and ammunition
mpsmithy= full armor
mpkohler= full ammo
mpstockpile= full ammo
mpbeamme= beam player to level start with initial conditions
mpsixthsense= no clip on/off
mpicu= toggle third person mode
mptachometer= show speed info on/off
mpgrs= show rotation on/off
mpgps= show position on/off
mpsizeme= show size info on/off