This page explains the requirements for unlocking the National Pokédex in Pokémon Brilliant Diamond and Shining Pearl.
National Pokédex Requirements
In Pokémon Brilliant Diamond and Shining Pearl, to unlock the National Pokédex, you need to have seen pokémon 001 through 150 in your Pokédex. You will see most of the pokémon in the Sinnoh Pokédex if you battle all of the trainers on the routes that you take during the story. See below for pokémon that are not encountered in trainer battles. For a list of all pokémon's locations, see List of Pokémon Locations in Brilliant Diamond and Shining Pearl.
Riolu can only be obtained by helping Riley on Iron Island, which you can reach by talking to the sailor next to the boat in Canalave City.
After you defeat the Elite Four and Champion, you can go to Celestic Town, enter the large building in the north part of town, and talk to the elder to add Dialga or Palkia (whichever one is not available in your version) to your Pokédex.
Azelf, Mesprit, and Uxie
After you defeat the Elite Four and Champion, you can catch Azelf, Mesprit, and Uxie. The following pages explain how to find and catch them.