Unless you have pkmn ranger and have completed the manaphy mission you can see it in a book in Backlot's office.
To receive a Manaphy in Pokemon Platinum, you must send over a Manaphy Egg from the game Pokemon Ranger after completing a special mission involving Manaphy.
They are in the maniac tunnel.
You can only find Darkrai through a Pokemon Event.
battle island
you don't get them together you have to find the sea crown and get manaphy from Pokemon ranger
you get the manaphy egg in Pokemon rangers and send it into Pokemon diamond or pearl
You can not find or get manaphy on Pokemon diamond but you can by hacking, trading or... if you have Pokemon ranger you have to complete the manaphy mission and then you can put it onto diamond.
To get a Manaphy in Pokemon Diamond you have to get Pokemon ranger and defeat it and you have to trade it to Pokemon Diamond,Platinum or Pearl.
There is no trainer in Pokemon Platinum with a Manaphy. However, you can trade with someone who has a Manaphy, or complete the special Ranger mission in Pokemon Ranger and transfer the Manaphy egg over to Pokemon Diamond, Pearl, or Platinum.
underground. look for an explorers pack in Eterna city.
info on manaphy in Pokemon mansion. trade for manaphy, and breed with ditto. you get a phione
you find a egg in the surronded sea and the next day it hatchs into manaphy