Level select
At the level selection screen, press C-Up, C-Right, C-Down, C-Left, R, L, Z, C-Up, C-Right, C-Down, C-Left. If you entered the code correctly, you will hear a sound.

All weapons
At the level selection screen, press C-Left, C-Down, C-Right, C-Up, Z, L, R, C-Left, C-Down, C-Right, C-Up. If you entered the code correctly, you will hear a sound.