Victini is the Pokemon Black and White special hidden/secret Pokemon who can not normally be captured within the game; Just like previous special Pokemon such as Mew, Celebi and Deoxys.
In order to find Victini, you will need to connect to the Internet via Nintendo Wi-Fi Connection starting March 6th and ending April 10, 2011. Only during this time frame will you be able to download the Liberty Pass item which is the key Event Item needed to unlock Victini. If you miss your chance to catch him, you may not ever get another chance (officially).
Once you have downloaded the Liberty Pass item, you’ll need to reach a certain point in the game before you can use it to catch Victini. Don’t worry if you haven’t reached that point yet, you can download the item from the beginning and still use it later on, as long as you don’t use it after the April 10th deadline. Or then it won’t work.