how do u get a national dex in pokemon diamond?

Answer 1

If you have battled everyone in the game, and seen Unknown, went to Mesprit, and got Rioli from Iron Island, plus beaten the Elite Four, then you should have seen all 150 Pokemon. You don't need Manaphy or the Nitendo Event Pokemon.

Oh, once you've seen 149 Pokemon, go to Celestic Town and see the old lady. She'll give you the picture of the last Pokemon, Dialga/Palkia, unless you traded.

Okay, now that you've seen all of them, go to Sandgem Town and talk to Professer Rowan. He'll congragulate you and give you the National Pokedex, plus some other things!

Answer 2

You have to seen all 150 pokemon in the Sinnoh Region. You have to battle everyone you come across to do it. Palkia's picture will be shown by Cynthia's grandma. Beat the Elite 4, go to the professor's lab and professor Oak will give you the National Podedex.
You don't have to see Manaphy to obtain it.

Answer 3

all you have to do is see all 150 pokemon from sinnoh. you can see a lot from the battle tower after you beat elite 4.(after you beat elite 4, talk to sailor at snowpoint)