If you want to start in the sixth level, you have to input A X A Y A X SELECT
SELECT SELECT START, when the boomerang cricles the word "START" in the start
menu.

If you want to choose you levels by your own, you have to press X in the
start menu (often and fast !).

A joke is also in the game, because when you press SELECT A A A A A A SELECT,
you play the old Atari Pitfall version.