To unlock all of the songs in the game automatically, hit the following buttons on your guitar at the title screen: Red, Yellow, Blue, Red, Red, Blue, Blue, Red, Yellow, Blue. If done properly, all songs will be accessible for play. Keep in mind, however, that this will disable your ability to save your game.

Addition from Dsva-0no - You can also unlock all of the songs by using the Wiimote. Insert the following cheat on the Wiimote at the game's main menu: B, 2, 1, B, B, 1, 1, B, 2, 1.