Thats easy go to route 221 go all the way to the end talk to the guy and you'll migrate the pokemon in you party from any game to diamond.(Note:only pokemon game and you must insert the game in your DS)
Finish the pokedex and the guy at the pal park will let you migrate when you have a game in the gbs slot. Peace.
Put a game boy game into the ds slot at the bottom and put pokemon platinum in the ds slot, start it up do not press continue game, go down until you find something that says what ever game you put in migrate/download, you can only migrate pokemon once a day with 6 Pokemon in total!! Then go to the pal park when you have your national Pokedex go in there and get catching!!!