In Pokemon Black for the DS, you need the Liberty Pass Mystery Gift in order to get to Liberty Gardens to catch Victini.
To get the Liberty Pass, you need a WiFi connection setup on your DS. You first need to download the mystery gift from the WiFi connection on the main menu.
Once you get your mystery gift, you need to go to any Poke Center in the game and speak to the delivery man dressed in blue. He’ll give you your Liberty Pass.
The Liberty Pass will only be available in the US until April 10th, 2011 so get it while you can.
Check out my other guide if you need information on how to catch Victini.