Pokémon Ranger: Shadows of Almia allows you to download very rare special and extra missions by connecting to Nintendo Wi-Fi Connection. Two new extra missions are now waiting for you on Ranger Net: "Dialga in Hia Valley!" and "Palkia in Haruba Desert!". To play these extra missions, you'll need to play through the game until you reach the end.

To receive these extra missions, go to Ranger Net from the main menu and download them via Nintendo Wi-Fi Connection. Unlike special missions, extra missions do not allow you to transfer Dialga or Palkia to your copy of Pokémon Diamond or Pokémon Pearl.

Source: http://www.pokemonranger.com/