1st you gotta complete the game and have the nationel pokedex then make your way to pal park (once you've been there once you can fly there) prof oak will appear saying its finally opened go in and speak to the man at the counter
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get firered or leafgreen and migrate it to pearl after national pokedex
Sorry, but you can't. You can trade from LeafGreen to Pearl, but not back.
On Pokemon Pearl's title screen, if the Pokemon LeafGreen game is in the GBA slot, click on "Transfer From LeafGreen" and you can choose six Pokemon from LeafGreen to transfer to Pearl, but be careful-you can't trade them back.
You have to migrate the pokemon so any pokemon you have in your leafgreen can be in pal park but it disappears from your leafgreen
it is not possible to trade Pokemon with DS and GBA. you may if you have diamond, pearl, hg, ss, platinum you can MIGRATE Pokemon from GBA games to DS games. this can be done in the main menu under "Migrate From (name of your game)" example: Migrate From Firered or Leafgreen.