How is the Sinoh Pokedex upgraded to the National Pokedex in Pokemon Diamond and Pearl?
Picking up the National Dex
Once you've seen the first 150 Pokemon in Sinnoh, you can upgrade your Pokedex to the "National Pokedex" where there aren't 150 slots to fill, but over 400. Now, simply head to Sandgem Town, one of the first locales, and the hometown of Professor Rowan and his Pokemon research lab.