Snorlax can't learn Explosion. Did you mean Self-Destruct? If so, here's how you learn it:
Step 1) Get HGSS and a PokeWalker. Go to Winner's Path, and walk 8000 steps (6000 with a Dark, Electric or Fighting Type), and catch a Munchlax. All Munchlax caught know Self Destruct.
Step 2) Transfer that Munchlax from the PokeWalker to HGSS, HGSS to Gen5, and Gen5 into Gen 6.
Step 3) Evolve Munchlax into Snorlax. Now, you have a Snorlax with Selfdestruct.
Optional Step) Breed Munchlax over and over until you get a female Snorlax/Munchlax. Now, you have a carrier for Self-Destruct for other Snorlax/Munchlax since Female Pokemon can pass down egg moves in Gen 6.
Oh, one more thing. You could have only recieved the Winner's Path from May 6 to June 25, 2010 over WiFi. Otherwise, you could test the GTS for luck or you're out of luck.
Note: Only Snorlax can pass down the move.
Source 1 | Source 2