How do you migrate Pokemon from Pokemon emerald to Pokemon Diamond?
Turn off your DS and put a Pokemon game into the gba slot. On the Pokemon menu screen there should be a new option, "migrate Pokemon from game in gba slot (ruby, shapphire and so on). Click on it, and choose the 6 Pokemon to migrate(has to be 6). Keep in mind that Pokemon can't have HMs, and they can't be returned to the game they migrated from.
Then go to Pal Park and enter the catching contest. The Pokemon that you migrated will all be in the park.You also need the national dex by seeing all the Pokemon in Shinnoh. The pal park is at the end of route 221.
(you will be given pokeballs that automaticly catch the Pokemon there so no worry about buying them)
hope this helps!