There are a few ways to get them into your game, but they all involve trading from another game:
* Handel: You can trade a Whismur from a Ruby, Sapphire, or Emerald game to your copy of Pokémon XD.
* Trådløs adapter: If you have a Game Boy Advance and a wireless adapter, you can connect to other players and trade Pokémon, including Whismur.
Unfortunately, there's no way to find or catch a Whismur in Pokémon XD itself.