On the title screen where it says "Please Set Side B", eject the disk and reinsert Side A again. A black screen will come up that saying "Please Set Side B". On Controller 1, press A, B, Up, Down, Right, Left. Press Start on the level select to play the game as usual (this time switching to Side B when asked). During the game, press A on Controller 2 to get the level select. Pick from 00 to 99 with the D-Pad on Controller 2, confirming with A.