You can't. You have to trade it in from another game.
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The Pokemon Misdreavus does not occur in the wild in Pokemon Platinum. The only way for the player to get a Misdreavus in this game is to have one traded over from another game.
You can't get Misdreavus in Pokemon Platinum, you have to trade it through Wi-Fi or get it from someone who has Pokemon Pearl. If Pokemon Pearl can't be found, go to McDonald's as they have free Wi-Fi.
you can get it in the southeast part of eterna forest
misdreavus does not level evolve into mismagius it evolves by using a dusk stone
easy battle the ghost type gym