In pearl-diamond when do you get the national dex?

You get the National PokéDex after seeing the 150 Pokémon in the Sinnoh Dex. After that go to Professor Rowan in his lab in Sandgem Town and talk to him and then Professor Oak will come in and present you with a National Dex as a gift.

If you have been looking around on the internet (more directly www.serebii.net) they will tell you that you need 151 (the +1 being Manaphy), Manaphy is only required in Platinum. -TyCundy