Press A(2), Up, B(2), Down, A, B, Start at the credits screen.

Press A(2), Up, B(2), Down, A, B(2), Start at when the phrase "Press Start" appears.

Press Left, Up, Right, Down, Up, Right, Left, Down and one of the following:

buttons at the credits screen.

Level - Button:

2 - Up
3 - Left
4 - Down
5 - Right
6 - Select
7 - B