Players must bring their DS equipped with an English copy of Pokémon Diamond or Pokémon Pearl to the DS Download Station usually located in the game section of any Toys R Us. Also, players must be sure they have already obtained the Pokédex and that there are no more than two Wonder Cards in their inventory, otherwise they will have to delete one of these Wonder Cards.
There is a limit of one Regigas per Pokémon Diamond or Pearl game card. The promotion will run from March 8th until March 21st at Toys R Us.
For those living in Japan, there is an alternative way to obtain Regigas (known there as Regigigas). Those who purchased a movie ticket to the latest Pokémon movie release will be able to download him for free from a Download Station.
According to information on the Japanese Pokémon site, trading Regigas to a Pokémon Platinum cartridge will make something special occur. It also allows players to obtain the Regi Trio (Regirock, Registeel, and Regice) in Snow Point City, the location players originally went to get Regigas in Diamond and Pearl.
They need to do give Darkrai out again. I didn't have a Internet connection at the time and didn't know about the promotion.
And when are they gonna give us Arceus?
One of the reasons why I don't like Pokémon games is because they aren't self-contained, you can't complete the game 100% without communicating with another version or with a special event like this. At least with the DS versions you can trade over Wi-Fi, but still, there's no way to obtain them all through in-game events.