First, you must of beaten the Elite Four before anything.
Than, you must prepare two DS's, and you must have your Black and White version character go to this lab, east of Opelucid City or somewhere along the after game routes.
I'm not exactly sure what you must do in your Platinum in order to activate the link, but your Black/White must have you speak to the professor up the stairs.
I apologize for not knowing all the details, but you may import up to six Pokemon each time, with no restrictions as to how many times you can migrate.
Hope this helps in some way ~
