You can't
How You Get A Manaphy Egg Without Action Replay On Pokemon Platinum?
You don't!
Http://www.psypokes.com/dex/psydex/490/all this has pretty much everything about manaphy
You go to the top of mt.cornet
You go to pokemon mansion
You need to get one on Pearl or Diamond in a special way then you can trade it over to Platinum.
No one knows anything, sheesh, there is a way, but I'm not talkin to any of you'll