By Action Replay, Or trade it from Pokemon ranger
The last Pokemon in the sinnoh dex is Giratina, and the last one in the national dex is Manaphy.
Darkrai
You have to see all the Pokemon in the sinnoh dex not including manaphy
150 on sinnoh dex 151 if you count manaphy
You can only catch Manaphy by transferring with 1 of the Pokémon Ranger games but if you only want Manaphy for your Sinnoh Dex in Platinum then there's a book in the Pokémon Mansion in Mr. Backlot's room that will add Manaphy to your Sinnoh Dex in Pokémon Platinum.
You cannot find Manaphy anywhere within Pokémon Platinum in order for you to catch it but if you're looking for its data in order to add it to your Sinnoh Dex as being "seen" then there's a book in the Pokémon Mansion that is near Hearthome City that will add Manaphy to your Sinnoh Dex for you so that it counts as being "seen" towards your Sinnoh Dex "seen" completion.
the Pokemon on the sihnodex after dialga and palkia is manaphy go to mr blacklot's room read the book and manaphy will be add to the dex
In Pokemon Platinum Version, there are two Pokedex'es, the Sinnoh Dex, and the National Dex. In the Sinnoh Dex, the 151 Pokemon is Manaphy. In the National Dex, the 151 Pokemon is Mew.
mew for national and Manaphy in the sinnoh dex
It is manaphy.
#151 of the Sinnoh 'dex is Manaphy, you can see it in the Pokémon Mansion, reading the book.
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.