First, you have to get the National Dex. Then you have to talk to owner of the Pokemon Mansion. He will say a random Pokemon that appears, so remember to save before you enter the room he's in. once he says that there's an Eevee, go out to the Trophy Garden and use the Poke Radar. It may take a while, though. In order for him to say any Pokemon you have to agree with him both times that you are envious of him and his garden. It worked for me in my Pearl. Even better, it was a shiny female.