You need to tune the Pokegear radio into the "Poke Flute" channel, which is at the top center of the circular tuner.
Then press A to "talk" to Snorlax and he will wake up and attack! It is level 50, by the way.
It's a good idea to bring a bunch of Ultra Balls, or Dusk Balls if at night, or Heavy Balls (made from Blk Apricorns) if you have them.