Pause game play and press L + R.
Enter LVDYK as a password and return to the title screen. Highlight the one player option, hold Left + L + R and press Start. Press X, A, L, and R to select a level, and Start to begin game play.
Level 6: SMPSN
Level 11: CVFTZ
Level 16: BPQGY
Level 21: TRQCZ
Level 26: CQQSH
Level 31: STFVF
Level 36: DJZZZ
Level 41: BLBJN
Level 46: DFLTQ