Successfully complete the game to unlock Centipede at the 'Atari Games' menu. If it is not unlocked as soon as you complete the game, restart, reload your game, and recheck the 'Atari Games' menu.
Defeating the Infiltrator
This Boss battle will be your mission in level 5. Press the buttons as instructed by the bottom captions. Do this at random so that the Infiltrator cannot predict your attack. When his health is down by 24, he will get a limp leg which will slow him
down.
Missile Command Mini-game
In the vet's office on level 13, walk into the small office on the far right side. Approach the computer and press [Action] to play Missile Command. Then, successfully complete the level to unlock the game at the 'Atari Games' menu.