To be able to access early unlockables and hidden features in the game use the Right analog-stick to look down at your hand when you are locked up at the 'Title' screen. Then keep pressing LT + RT to escape the chair (this also unlocks the 'Just Ask Me Nicely' Secret Achievement) and move to the area behind your chair where you will find an old computer terminal that you can plug a USB keyboard into your Xbox 360 and type one of the following case sensitive codes (without the quotes) to enable the corresponding effect.
Unlock ALL Missions, 'Five' Zombie Mode Mission, Dead Ops Arcade Game, Zork Game:
Enter '3arc Unlock' as a code.
Unlock ALL Intel:
Enter '3arc Intel' as a code. Enabling this code will prevent you from getting the 'Closer Analysis' Achievement.
Display ALL Commands:
Enter 'help' as a code. This displays a list of system commands in the terminal and Pentagon user e-mail access.
Unlock Dead Ops Arcade Game:
Enter 'DOA' as a code. This feature is a zombie killing mini-game similar to Smash TV. Use the Left analog-stick to move and the Right analog-stick to shoot. Play this mini game to get the 'Insert Coin' Achievement.
Unlock Zork Game:
Enter 'ZORK' as a code. Play this mini game to get the 'Eaten By A Grue' Achievement.
Added by: Sanzano Nov 9th 2010, ID#6554