Enter all codes at the cheat entry screen:

Infinite Ammo - X, A, Y, Y, Y, A, X, B
Futuristic Weapons - A, A, A, X, B, X, B, B
God Mode - Y, X, B, B, A, A, B, X
Infinite Credits - B, B, B, X, B, A, Y, A

Successfully complete the game to unlock the classic Centipede game at the "Atari Games" menu. Note: If it is not unlocked as soon as you complete the game, restart your Xbox, reload your game, and recheck the "Atari Games" menu.

In the vet's office on level 13, walk into the small office on the far right side. Approach the computer and press A to play the classic Missile Command game. Then, successfully complete the level to unlock the game at the "Atari Games" menu.