Instructions
Attach the white dongle that comes with "Rock Band" into one of the USB ports on the back of the Nintendo Wii.
Hold down the guitar icon on the dongle until the light starts flashing.
Hold down the "Sync" button on the guitar. The "Sync" button is located under the whammy bar. When both lights stabilize, that will indicate the sync process has completed. Repeat these steps to sync the second guitar.
Use the buttons and strum on the guitar to navigate the "Rock Band" menu. When prompted, activate the second guitar (or bass) in the multiplayer mode of your choice.