You need someones friend code. You will then put it and their name into your pal pad. Then you will go down to the bottom floor of the pokemon center and go to the middle nurse joy. Talk to her and enter. Thats the way I know it works in platinum, for the other two I don't know. Hope that helps, if you need a demo you can send me a PM and I'll help you further.
At this point this should probably be locked.