(Story) Both Dialga & Palkia To The Pokedex Offline – Pokémon (BDSP)
Depending on what version of the game you have Dialga or Palkia will be registered to your Pokedex automatically during the story. Diamond: Dialga, Pearl: Palkia.
To resister the other one you will have to get in the hall of fame by finishing the Pokémon league!
First you need to go to Celestic Town.
Click Town Map option.
Enter in the house in the town.
Once in post game you will have to talk to the NPC in the house.
She will show you a photo of the other legendary.
Click Pokedex option.