Instructions
Walk, fly or ride your bicycle to return to Route 11.
Walk over to the sleeping Snorlax blocking the path.
Open you PokeGear menu and select the radio. Tune the radio to the PokeFlute station. This will wake the Snorlax.
Defeat the Snorlax in a Pokemon battle to clear it from the road and allow passage to both the cave and the other portion of Route 11 that the Snorlax was blocking.