You need to have seen all 150 Pokemons in the Sinnoh region to get this. You should have a total of 149 entries in your Pokedex when you complete the game, provided that you battled all the trainers in the game. The missing entry would either be for Dialga or Palkia (depending on which version of Pokemon you have).
Proceed to the into the large house at the north side of town. Speak to Cynthia's granny and she'll show you a picture of either Dialga or Palkia (depending on whether you are paying Diamond or Pearl). Now that you have completed the Sinnoh Pokedex, it's to get it upgraded.
Proceed to Professor Rowan's Research Lab and show him your completed Pokedex. Rowan will reward you with the POKE RADAR application for your Poketech. Professor Oak will come in and upgrade your Sinnoh Dex to the National Dex.
Keep reading the following chapters for details on how to complete the National Dex.