Instructions
Go to Lavender Town after defeating Misty at Cerulean Gym. It is connected to Saffron City by Route 8. You can get the item needed to wake Snorlax before this, right after repairing the power plant, but defeating Misty first saves your having to backtrack to Cerulean City later.
Talk to the man outside the radio station. He gives you the Expansion Card as a reward for fixing the power plant.
Return to where Snorlax is sleeping on Route 11.
Open the Pokegear menu. Select "Radio."
Choose the Pokeflute Channel from the map. This is not one of the radio station icons, which can make it easy to miss. It is located just north of Station 1 and Station 2 on the map. Snorlax wakes up after the music starts playing from this channel.
Talk to Snorlax to start a fight. This is the only opportunity to capture a Snorlax in "Pokemon SoulSilver."