How You Get A Manaphy Egg Without Action Replay On Pokemon Platinum?

You can't

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.