You don't, Mr. Backlot doesn't show you Manaphy but there's a book in his room that will show you a picture of Manaphy as well as add it as a "seen" Pokémon to your Sinnoh Dex.
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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.
In the Platinum pokedex, number 151-or the Pokemon after Palkia-is none other than Manaphy. Manaphy can be obtained after you beat the manaphy egg mission in Pokemon rangers. You then can transfer it to platinum via mystery gift. what you do you go to the Pokemon mation and go to the oner and behind him theres a book talk to the book and it will show you a picture of manaphy
you cant catch manaphy unless you have Pokemon ranger and do something with that,but you can see it.go to celestic town and read the book on the table in the big house it will show manaphy,like in diamond it shows palkia and in pearl it shows dialga.
well u have to see manaphy. in order to get manaphy on your pokedex, go to the library in Canalave City. On the top floor i think there is a book on the table. Read it and it will show you a pic of manaphy and it will automatically be added to your dex. also if you need help on finding the other Pokemon to complete the sinnoh dex go to this link-----------> http://pokemonplatinum.net/v2/pokedex/
You can buy them in Sunnyshore City or show unknown to the boy in Solacean town.