It is the same as diamond and pearl. Go downstairs at any Pokemon center and talk to the lady closest to the stairs. They won't allow you to go down there until a certain point in the game. I think it is when you beat Roark, the first gym leader, and get the coal badge.
Chat with our AI personalities
You can trade Pokemon with friends in the Pokemon centre,but you need a pal pad first
Depending on if you have gotten your Pal Pad yet determines when you can see your friend code. To get your pal pad, walk into any Pokemon center as long as you have your first badge and go to the basement. There a lady behind the counter will register you the item "Pal Pad." To see your code you must open up your items and go all the way to the key items pocket. Scroll down till you find pal pad then click register or use. Then click "My Friend Code" then vola! I hoped that helped. If not open up your Pokemon Platinum Instruction booklet that came with the game and see page 42. :] But you need to log on the Nintendo WFC and I dunno how to do that!!!
You cant find it in platinum. Trade it from diamond or from a generation III game via pal park.
To get your emerald Pokemon to platinum you transfer them in pal park.
it ispossible i have Pokemon platinum on my R4 and i migrated Pokemon from Pokemon ruby and it worked but first u need the pal pad you can get that when you have finished the sinnoh pokedex.