The special "Liberate the Tower" Wi-Fi mission for Pokémon Ranger: Shadows of Almia is now available for players in both North America and Europe. Once you complete this mission, you are able to obtain a special Darkrai that can be transferred over to your Diamond or Pearl cart. The mission can be accessed now by entering Ranger Net. The mission will be available from December 1st, 2008 to January 30th, 2009, so be sure to access the mission now while you can.

Pokémon Ranger: Shadows of Almia was released in North America on November 10, 2008. The "Liberate the Tower" mission is the third special mission that has been made available via Ranger Net.

Source: http://www.pokemonranger.com