Either migrate it or get a ditto and press ab ab at the same time then put him in the day-care then don't go into any poke-center or any building don't leave the town and save turn off your ds turn it on then ask the day-care man and he says your ditto seems to have had a egg
Transfer it from pokemon ranger
I didn't see the man in green at the poke mart tell me how to get Manaphy on platinum
Get pokemon ranger,do the manaphy mission,tap on the egg and choose send to pokemon pearl/diamond,turn on pokemon pearl/diamond and choose mystery gift from the game menu then choose get via wireless after that go to any poke-mart(apart from the pokemon league and veilstone department store) and talk to the man in green by the counter(also have a spare slot in your party) and get the manaphy egg then all you have to do is hatch it.
You need 2 DS 1 pokemon ranger in it I btw you need 2 beat the managy mission on it so then you have this game on the other ds and then I am not sure about the rest I no it's a egg for sure trust me all the answers up there r wrong just google "who to get ------ in pokemon platinum or what ever the game is
After you beat the elite four go to snow point city and go to the sailor near the boat and he'll take you to the fighting area and go to the poke mart and talk to the man in green next to the counter and he'll give you a manaphy egg.