On Mercay Island, go north of the port. Go up the northern ramp, and scratch your head about the small isle next to you. Go west but not to the Temple. Follow the ridge to a wall of rock that stops you. Blow it up with a bomb. Go in the opening, and cross the seemingly endless bridge. Exit to the small isle, and hop off the small ledge to a switch. There appears a bridge. Go up the few ramps to a guy with a guitar by the name of Freedle. Talk to him and unlock Tag Mode!