If you've beaten the elite 4 and have every Pokemon recorded in your pokedex (dialga, palkia, giratina, rotom, etc. included) you need to talk to Prof. Oak in Celestic Town to get the national dex
Pokemon Platinum has two different Pokedexes: the Sinnoh dex and the National Pokedex. In the Sinnoh dex, #152 is Rotom. In the National Pokedex, it's Chikorita.
you see al the Pokemon in sinnoh
You have to find all of the Pokemon in Sinnoh.
you have to see all the Pokemon in sinnoh
go and see all the Pokemon in the sinnoh 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.
You can catch and find all the sinnoh Pokemon and when you see all the sinnoh Pokemon you get the national dex and find and catch all the pokeradar Pokemon.You get the pokeradar when you get the national dex.
Cranidos in the Sinnoh Dex, Fearow in the National Dex
First complete the sinnoh pokedex, 210 Pokemon
National Dex: Igglybuff Sinnoh Dex: Togetic
spiritomb in the sinnoh dex lickitung in the national dex
i think national is Omanyte and sinnoh is Feebas