You have to beat the Pokemon Leagues to do this. Go to Canalave City and go to the sailor's house. There will be a little boy sleeping in his bed. Talk to his mom and then talk to the sailor outside. He will ask you to go to Full Moon Island where Cresselia is to get something to wake the little boy up from his nightmare. People who has nightmares in Canalave City could not wake up. When you go up to Cresselia, she will fly somewhere else just like Mesprit.
You cannot catch Celebi in Pokemon Diamond.
yes, you can catch Pokemon in Pokemon diamond
neither of them is better, they are both the same, but in diamond, you catch dialga, and in pearl, you catch palkia, and you can catch different Pokemon in each game for example: you can catch this Pokemon in diamond, but not pearl but: you can catch this different Pokemon in pearl, but not diamond
You can catch makuhita in the fighting area in Pokemon diamond
How do you get a cherrim in Pokemon diamond.
ucant catch misdreavus in diamond
You cannot catch a Metang in Pokemon Diamond. To get one, you can evolve a Beldum.
You can't catch a Lugia anywhere in Pokemon Diamond.
Nosepass is a swarm Pokemon on Pokemon diamond
you can catch a pikachu in the Pokemon mansion.
You can not catch a manaphy on Pokemon diamond. you must trade it somehow on a Pokemon rangers game.
No, you can't catch Deoxys on Pokemon Diamond & Pearl unless you have cheats. You can transfer it from your GBA to Pokemon Diamond and Pearl though