Sorry, you cannot obtain Jirachi in Diamond Version without the use of cheats.
You can obtain it via Pokemon Colosseum pre-order disc though.
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.
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.
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.
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.
if you got cheats on diamond etc. you can trade it
yes you can but the event is over now i got it though :) o btw its only for pearl platinum and diamond
catch it at the place you caught shaymin after you have the azure flute and then you play it