When you are at the main menu look down with the right analog stick and button bash the trigger and the ADS button. You will now be out of the chair. When you are go behind the chair and enter these codes into the computer.
3ARC UNLOCK - Unlocks the zombie map "FIVE" and all the campaign missions.
3ARC INTEL - Unlocks all the mission and all Intel (Stops you from getting the Trophy for it)
ZORK - You get a 15Gs Achievement for this and you get to play a word game (Its bad)