The full instructions for the event are available at Pokemon.com, along with this information:
You’ll have a new opportunity to get the Legendary Mewtwo once again! Between February 12th and March 6th, you can receive Mewtwo in your Pokémon Black Version or Pokémon White Version game. Don’t miss this limited-time distribution event to get this amazing Pokémon via the Nintendo® Wi-Fi Connection service (wireless broadband Internet access required. For more info, go to support.nintendo.com).
When you take this Level 70 Mewtwo into battle, know that you have one of the strongest Pokémon ever. Its moves are powerful and well suited to take advantage of Mewtwo’s strengths and cover its weaknesses. Psystrike is a new signature move for Mewtwo that does damage depending on its opponent’s Defense. Ordinarily Mewtwo doesn’t learn Psystrike until Level 100, but this Mewtwo already has the powerful attack! Mewtwo also has Shadow Ball, a Ghost-type move that is super effective against the Ghost-type Pokémon that can cause problems for Psychic types. Mewtwo is also weak against Dark-type Pokémon, so its third attack, the Fighting-type move Aura Sphere, is a good counter to their attacks. Lastly, this special Mewtwo knows Electro Ball, a stunning Electric-type move that Mewtwo cannot normally learn!
When you get Mewtwo, it will be holding a King’s Rock, an item which occasionally makes its opponent flinch in battle. You can also give it to certain Pokémon and then trade them to make them evolve. Plus, Mewtwo comes with a Premier Ribbon, a special Ribbon that you can’t get through normal game play!
Mewtwo will be available via a Nintendo Wi-Fi Connection distribution event that runs from February 12th through March 6th.
A reminded, this is not an in-store event, but a download available over your local wireless internet connection to Pokemon Black or White only. For instructions on setting up that connection, go to the US Nintendo support site.