Krazy Ivan Codes -

Level select:
Highlight Russia and press Right at the level select screen with the globe. Hold X + Down/Left before the Japanese mission details appear. The globe will begin to bounce through the levels. Release the buttons when a specific level is displayed to play that stage.

True ending:
As the shield generator is blown up on the final level (Japan), walk into the center of the explosion for a hidden level and the true ending of the game.