While you are at the Main Menu, use the right analog stick to look down to your hands. You will see you are locked up. Then keep pressing LT and RT to escape from the chair (this will also unlock the "Just Ask Me Nicely" secret achievement!

Once you are free from the chair, move around to the area behind the chair and you will see a computer terminal that you can use. Then simply enter in one of the following codes!

• Enter "3arc Unlock" as a case-sensitive code to unlock the "Five" Zombie mode mission, Dead Ops Arcade game, Zork text-based adventure game and all missions.
• Enter "3arc Intel" as a case-sensitive code to unlock all the Intel (note, enabling this code will prevent you from getting the "Closer Analysis" achievement).
• Enter "DOA" as a case-sensitive code to unlock the Dead Ops Arcade game. This is a zombie killing mini-game similar to Smash TV! The left analog stick moves you and the right analog stick shoots. If you play this game you get the "Insert Coin" achievement.
• Enter "ZORK" as a case-sensitive code to unlock the Zork text based adventure game. If you play this game you get the "Eaten By A Grue" achievement.
• Enter "help" as a case-sensitive code to display a list of the system commands in the terminal. You also get Pentagon user

From: Mega Daddy T at neoseeker forums.