Where can you see a manaphy on Pokemon Platinum?

In order to see Manaphy in Pokémon Platinum, you can go to Mr. Backlot's Room in the Pokéemon Mansion, you'll see on the table behind him is a book. Interact with the book and it will show you a picture of Manaphy which will add it to your PokéDex as being a 'seen' Pokémon.