First enter the level select code. Now,go to sound test option,and play the following tunes - 1, 9, 9, 2, 1, 1, 2, 4 - It should make a ring sound when track 4 is played to signify you've entered it correctly, now select the stage you want to go to, and press start while holding A until the stage starts,and you'll have debug activated.Here are the controls.

B-Turn Debug on/off
A-Switch object
C-Put down object selected

Pressing A while the game is paused will cause the game to reset.