I haven't seen a topic for this so I wanted to inform everyone before it's too late. Mewtwo is available as a Mystery Gift on Pokemon Black/White.
The promotion started almost a week ago to celebrate something related to the TCG. However! It isn't too late to get Mewtwo since this promotion ends on the 26th of this month.
To download Mewtwo, just boot up your game. After making sure you're connected to the internet, go to Mystery Gift and then 'Recieve Gift'. Get it via Nintendo WFC and it will search for the gift. Once you see "Mewtwo Returns" just press A and you download Mewtwo!
You'll receive him in-game through a deliveryman in all the Pokemon Centers, and he's all yours. He starts at level 75.
Here's my source btw: http://www.destructoid.com/psa-get-mewtwo-for-pok-mon-white-and-black-221305.phtml