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