You have to get the national pokedex then go to the guy who brings you to iron island and he'll ask you to help wake up his kid by going to crescent moon island. You will find Crescellia there and he will do the same as mespirit.
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on crescent moon island in canalave city after you beat the game
there is no fire island on platinum there is full moon island and crsent moon island (were you catch cresilia and darkria) and iron island that is it
New moon island is only avalible in Pokemon pearl, diamond and platinum
Complete the main story and complete your pokedex then head to Canalave City. Go inside the house that is near to the sailor that takes you to Iron Island. There will be a boy who's stuck in a Nightmare then talk to his mom.Go outside and talk to the sailor and he will ask you a favor to go to the crescent moon island and he'll take you there.
iron island, (where you meet riley and he gives you a riolu egg,) Full moon island,(where you first see cresselia,) and New moon island (where you catch darkrai)