Level Select
     Pause  gameplay,  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.

Extra Levels
     Pause  gameplay, hold Z and press C-Down, L, R, L, R, L, R, L, R,
L, R. Now go to the Enter Secret Worlds option and press A.