You cannot find Jirachi in Pokémon Diamond. Jirachi is only available during limited time distributions or via dowloading it from the Pokémon Colosseum pre-order bonus disc.
catch it at the place you caught shaymin after you have the azure flute and then you play it
You cannot get a Jirachi in "Pokémon Diamond" except for through using the Colosseum Bonus Disc, downloading the Jirachi from that into either "Pokémon Ruby" or "Pokémon Sapphire" and then migrate it into Pokémon Diamond or you could have gotten a Draco Meteor Jirachi from the Gamestop Jirachi event that was held in February and March of 2010, additionally you can use an Action Replay cheating device or a Pokésav in order to cheat a Jirachi into Diamond.
you have to go to the unkown place and defeat 100 unknown in a row without losing the same with jirachi
Jirachi cannot be caught in Pokemon Diamond. It can, however, be transferred from Ruby, Sapphire, or Emerald, assuming you got it from a Nintendo event.
Nowhere. It can only be downloaded or migrated.
Sorry, you cannot obtain Jirachi in Diamond Version without the use of cheats. You can obtain it via Pokemon Colosseum pre-order disc though.
The only way to get Jirachi in Diamond Version is to import it through the Pal Park from either Ruby or Sapphire Version. In either Ruby or Sapphire, though, the only way to get Jirachi in the first place is to play either Pokemon Channel or Pokemon Colosseum and import it from there.
i think you can only catch it in Pokemon emerald.
An event or trade from another game
Yes. Use an Action Replay
Oak's letter is in diamond, pearl and platinum and it doesn't give you jirachi it gives you shaymin.
You must get Jirachi from an event in Diamond, Pearl, or Platinum then trade to heartgold. There was also a Wi-Fi event in which you could receive a Jirachi which has since passed.