Pal Park gets the pokes on your game card. It's at the end of route 221, and is a flying location.
Just get the pokes you want to trade (I always put them on emerald and clone) and scroll down to Migrate from (GBA game). The pokemon cannot know an HM as you select them. Migrate, confirm that they'll be removed etc. Go to your D/P save file, Fly to Pal Park, go in catch your pokemon and save and have fun with ur old GBA pokemon.
There's a better guide in the D/P FAQ, Second post