From the main menu go to options then down to cheats go to enter cheat. When you are entering the cheats you must hold the frets (G,R,Y,B or O) and then strum them in this specific order:

G=green fret R=red fret Y=yellow fret B=blue O=orange.

(GR_BO) (GRYB_) (GRY_O) (G_BYO) (GRYB_) (_RYBO) (GRYB_) (G_YBO) (GRYB_) (GRY_O) (GRY_O) (GRYB_) (GRY_O)

Note: no sounds will play when you strum these chords.