Enter these codes in the Keys Section.

All Accessories    - STXOXX TSXO SOX TOY
Acid               - STOS TSX TX TOX
All Keys           - STXTO TX TXTSX MIK 
Toggle Cloak       - XTO SOTSX TXTO RED
Extra Life         - TXSO TX SXO XTO +1 UP
Full Sheilds       - TXOS SXO TSXOS BUG
Robots Move Faster - TXS STO SXO STO JAVA 
Turbo              - TSOX SXO STOXX SVT 

During the game press: Triangle, X, Triangle, Circle, X, Triangle, Square, X, Triangle, X, Circle, Triangle.

On level 2, when you destroy the reactor go to the opposite side of the exit. Shoot the wall and you should see a red wall. Enter it and you will warp to the secret level.