You have to see all 210 pokemons or find the national pokedex cheat code at http://www.cheatsguru.com/nds/pokemon_platinum/cheats/1828918.htm
. Enjoy.
PS I need one more Pokemon till I get the national pokedex!:)
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In Platinum dex' its Dusknoir in National dex' its Sunkern.
In Pokemon Platinum to get the National Dex you will need to have seen all 210 Pokemon in the Sinnoh Dex then talk to Professor Rowan and then after a while Professor Oak will give you the National Dex!
go and see all the Pokemon in the sinnoh dex
National Dex: Igglybuff Sinnoh Dex: Togetic
in sinnoh dex it is togetic but in national dex it is igglybuff