So you go to Game Stop. Then, you turn on your Pokemon Black/White 2 game. Select "Mystery Gift", then select "Receive Gift". Next, choose "Get Via Wireless". Next, you will receive your Dialga/Palkia/Giratina. This next part you can do anywhere. Make sure you have an open slot in your party, and you can pick it up at any Pokemon Center.
The events are in US fortunately. Dialga is Aug. 19 - Sep. 8, Palkia is Sep. 9 - Sep. 29, and Giratini is Sep. 30 - Oct 20.
Hope I helped! :)
Source: Here (dates) and experience (method).