* Lopunny is a Generation IV Pokémon, introduced in Pokémon Diamond and Pearl. While those games share a lot of content with Platinum, Lopunny itself is not found in the Platinum game.
* You can obtain a Lopunny in Platinum through trading. This is the only way to acquire one in the game.
If you're looking to catch a Lopunny, you'd need to play Pokémon Diamond or Pearl, or trade with someone who has one.