What else needs to be done to trade between Diamond and LeafGreen besides putting them into the proper slots on the DS?
In order to migrate your Pokémon from LeafGreen to Diamond, you first need the National Dex. Once you get it, you then need to go into LeafGreen and make sure the ones you want to transfer are all in the PC and then save the game. Next start Diamond and once you get to the menu that says "Continue, New Game," you will see an option that reads "Migrate from Pokémon LeafGreen" so select that option. This will take you into LeafGreen's PC. Select any 6 Pokémon and then you will confirm your selection as well as warn you that once you transfer the Pokémon, they cannot return to LeafGreen, also Pokémon that know HM moves as well as have Mail attached to them will not be able to migrate. After confirming, go into your Diamond game and head to Sandgem Town. Take the water route that is to the south and follow it until you come across a building, that building is the Pal Park. Enter into it and you will be given Park Balls after talking with the Receptionist and he will let you in. You will have to go through the various patches of grass and water spots in order to find your 6 Pokémon. Once you find a Pokémon, throw a Park Ball at it and it will be caught. Park Balls never miss. Once your capture session is over, you will be locked out for 24 hours.