To get infinite ammo, go to the Classified Files screen and enter this code: BLASTRADIUS.
To unlock God Mode, go to the the Classified Files screen and enter the code TEAMBUDDIES.
To unlock all missions, go to the Classified Files screen and enter this code: KINGSLEY.
To unlock the first extra mission, go to the Classified Files screen and enter this code: KREUZLER.
To unlock the second extra mission, go to the Classified Files screen and enter this code: SHEARER.
To unlock the third Extra Mission, go to the Classified Files screen and enter this code: UBERDOOPER.