At the main menu, select "Options", "Cheats", "Enter New Cheat", then enter one of the following codes to unlock the corresponding cheat option:

Performance Mode

Press Yellow(2), Blue, Red, Blue, Green, Red(2).

Hyperspeed

Press Green, Blue, Red, Yellow(2), Red, Green(2).

Auto Kick

Press Yellow, Green, Red, Blue(4), Red. The bass drum pedal will now be played automatically. Note: Enabling this code will disable earning any money after a song has been completed.

Unlock Quickplay

Press Blue, Blue, Red, Green(2), Blue(2), Yellow.

Aaron

Press Blue, Red, Yellow(5), Green.

Nick

Press Green, Red, Blue, Green, Red, Blue, Blue, Green.

Johnny Viper

Press Blue, Red, Blue(2), Yellow(3), Green.

Rina

Press Blue, Red, Green, Green, Yellow(3), Green.

Air Instruments

Press Red(2), Blue, Yellow, Green(3), Yellow.

Invisible Characters

Press Green, Red, Yellow(3), Blue(2), Green.

AT&T Ballpark

Press Yellow, Green, Red(2), Green, Blue, Red, Yellow.

Gem Color

Press Blue, Red(2), Green, Red, Green, Red, Yellow.

Flame Color

Press Green, Red, Green, Blue, Red(2), Yellow, Blue.

Star Color

Press Red(2), Yellow, Red, Blue, Red(2), Blue.

Vocal Fireball

Press Red, Green(2), Yellow, Blue, Green, Yellow, Green.

Always Slide

Press Green(2), Red(2), Yellow, Red, Yellow, Blue.

Extra Line 6 Tones

Press Green, Red, Yellow, Blue, Red, Yellow, Blue, Green.

Successfully complete the indicated task to unlock the corresponding character for purchase:

Billy Corgan: Play "Today" in Band Career mode.
Hayley Williams: Play "Misery Business" in Vocals Career mode.
Ozzy Osbourne: Sing "Mr. Crowley" and the encore song at the "Germany: Ozzfest" gig in Vocals Career mode.
Rockubot: Successfully complete Drum Career mode.
Skeleton: Successfully complete Vocals Career mode.
Sting: Play "Demolition Man" in Bass Career mode.
Ted Nugent: Win Ted Nugent's Guitar Battle in Guitar Career mode.
Travis Barker: Play "Dammit" in Drum Career mode.
Zakk Wylde: Win Zakk Wylde's Guitar Battle, and play the encore song with him at the "Louisiana: Swamp Shack" gig in Guitar Career mode.