Well first of course you need to see all 150 Pokemon from the Sinnoh Pokedex. Don't freak out if you don't know how to get the opposite Dragon (Dialga and Palkia) Talk to the woman in Celstic Town after beating the Elite 4 and she will tell you about Dialga/Palkia, it will get added to your Pokedex. OK once you seen all 150 Pokemon go to Sandgem town and talk to Professor Rowan. Then Oak from nowhere barges in and gives you the National Dex!!!
