This is one of the special Mystery Gift events for Ranger. Sadly, you cannot send the Giratina to your Diamond, Pearl, Platinum, HeartGold, or SoulSilver games. The story goes that some people in the past have stolen the Griseous Orb. You must help the Giratina get the Griseous Orb back so it may return to its homeland. You can't play this mission until you beat Story Mode. This event doesn't have an ending date so you have plenty of time to get this mission. Soon, we will probably find out about the Arceus mission too.
Source:
http://www.pokemon.com/us/news/vgran...na-2010-12-13/