Simply follow these instructions to obtain either Mega Stone, Mewtwonite X or Mewtwonite Y, and evolve into Mega Mewtwo X or Mega Mewtwo Y:
- Select Mystery Gift on the main menu.
- Select Receive Gift.
- Select Get with Code/Password, then Yes, and then Yes again to connect to the Internet.
- Enter the code M2DESCENT
- Watch as you receive Mega Stones!
- Speak to the deliveryman in any Pokémon Center to pick up your Mega Stones.
- Be sure to save your game!
For more information, please visit http://www.pokemon.com/us/pokemon-news/get-a-pair-of-mega-stones-for-mewtwo/.