Press ENTER next to the number pad to bring up the Console and enter these cheats. Press Enter again to activate them.

god mode: superman
Commit suicide: god
Infinite Ammo: ammo
Invisibility: shadow
Paintball Mode: extremepaintball
Refill inventory: refill
run faster: run
Specialist unlockhero
Specialists: unlockheros
Squirrel Launcher: squirrelkite
team god mode: teamsuperman
Team Invisibility: teamshadow
Win or lose current mission: autowin/autolose
Chicken grenades: chickenrun