To do this, you need two DSs. They can be 3DSs, or any other model you want. You just need two devices that can each play the two games. Put the games in the devices.
- Go to the Poke Transfer Lab on Route 15 in Black 2 (You can only go here once you've completed the game) and talk to the person at the desk, and you will start searching for a gen 4 game.
- Put the Pokemon you want to transfer from Platinum in any PC box.
- Save and close Platinum, then on that device go to DS download play (make sure you don't select 3DS download play if you're using a 3DS). Select the title that pops up.
- You can now select which Pokemon to transfer on the gen 5 game. You can only do 6 at once and you have to do a stupid mini game, but it's the only way of doing it.