Pokémon Black and White feature several event Pokémon, of which only three are available as of yet.
Victini
Victini, the first Fire/Psychic type Pokémon, is only obtainable via the Liberty Pass event, which runs until mid-April 2011. To receive Victini, a Wi-Fi connection is necessary. When you start up the game, select Mystery Gift from the menu screen, and choose to obtain a gift via Nintendo WFC to obtain a Liberty Pass. Pick this up in-game by talking to the delivery man in any Pokémon Center. Take the Liberty Pass to Liberty Pier in Castelia City and talk to the woman by the boat. After a scene involving Team Plasma proceed to the tower, battling some Plasma Grunts along the way, and enter. The room at the back contains one final Grunt and Victini. Bring plenty of Ultra Balls because this one isn't an easy catch.
Zorua
Zorua, a Dark-type Pokémon with an interesting ability, is only obtainable if you have the event Celebi from Pokémon HeartGold and SoulSilver. To transfer Celebi, you have to find a scientist in a building near Castelia City's northern gate, who will tell you about a wonderful device that only someone who knows the password can use. The first password is Everyone Happy; the second is Simple Connection. He will then give you access to his Relocator, which you need two Nintendo DS systems to use. To Relocate Celebi, select the Relocator option from the main menu on Black/White Version. It will give you the title of software you must download to the other Nintendo DS, which must have the Game Card containing the event Celebi inserted. Follow the on-screen instructions to transfer Celebi to Black/White Version. In-game, travel to the Game Freak building in Castelia City, with Celebi in your party, and a spare Poké Ball in your bag. There, Celebi will come out of its ball, causing a little boy to get excited. The little boy is actually Zorua in disguise, and will join your party if you have a spare Poké Ball on hand.
Zoroark
Zoroark is the evolved form of Zorua, but is also obtainable through a separate event. Follow the above instructions to Relocate an event Entei, Raikou, or Suicune to Black/White Version (the Roaming Pokémon ordinarily found in HeartGold and SoulSilver don't count for the purposes of this event). In-game, place the transferred Pokémon in your party, and travel to Route 16. Near the Nimbasa City Gate, travel north through the woods until you reach Lostlorn Forest. Eventually you'll find a caravan. After a short cutscene, enter it, and you'll be attacked by the woman inside, who is actually Zoroark in disguise. Unlike Zorua, you'll have to battle her to catch her, but be careful, since she's only Level 25. A good attack to use here is False Swipe, which never defeats a Pokémon; it always leaves it with no less than 1 HP. You'll need plenty of Poké Balls to catch Zoroark.
Keldeo
This event has not yet occurred.
Meloetta
This event has not yet occurred.
Genesect
This event has not yet occurred.