Level select
Pause the game, then hold Z and press R, B, A, B, A, C-Up, C-Down, C-Left, C-Right, C-Up, C-Down, C-Left, C-Right. If you entered the code correctly, you will hear a sound. Then, highlight the new "Unlock Worlds" option that appears and press A to select any level.

Bonus levels
Pause the game, then hold Z and press C-Down, L, R, L, R, L, R, L, R, L, R. If you entered the code correctly, you will hear a sound. Then, highlight the new "Enter Secret Worlds" option that appears and press A to play the new levels.