You have to get all 210 Pokemon in the sinnoh dex. Then go to Rowan's office and professor Oak will come in and give you the national dex.
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!
In Platinum dex' its Dusknoir in National dex' its Sunkern.
You complete the shinho dex first and then you get the national pokedex
The next Pokemon after weavile is magnezone in the national dex
you have to get the national dex.
in sinnoh dex it is togetic but in national dex it is igglybuff
You get a life lol
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one of them is to have the national dex.
Get the national dex
In the sinnoh dex: porygon In the national dex: sunflora
There are 210 in the sinnoh dex and 490 in the national dex