Answer:
Once you beat the PKMN league, you should talk to the Prof. and he'll give you a Nat. Pokedex. You will be able to get some Pokemon filled in it,but you have to have a previous Pokemon GBA game like Ruby, Emerald, Sapphire, Leaf Green, or Fire Red. Then you put the game pack into the game-pack slot.Then you'll need to find the Pal Park, and put some of your PKMN in the park. Go into the field and find your PKMN, and they'll be registered into your Pokedex.
You won't have a game pack slot for Nintendo DSI.