Forum Thread
Beedrillite
Forum-Index → Help → Answered → BeedrilliteInstead of having each Mega Stone [ex: Beedrillite, Gardevoirite, etc.] there's only one Mega Stone Item that you can win in Treasure Hunts in the Game Center, by boiling it in the Gem Cauldron or occasionally with trading through Leah at Emera Beach.
You also need the respective Pokemon with a "Mega-power" [shown here]. Pokemon can hatch with the Mega-ability randomly and only they can Mega Evolve.
After you get a Mega-able Pokemon, you have to give the Pokemon a Mega Stone as a held item and it will Mega Evolve starting at Level 50.