you have to wait until Nintendo anounces an event or get it from a friend
You can't
use the mystery gift
You can catch Dialga, Uxie, Azelf, and Mesprit
you have to get the arceus event on mystery gift.
You cant. Once it is there it is there PERMANETLY
Get the key from Mystery Gift and open the door in caveport that is locked.
The only way to get Oak's Letter as a Mystery Gift was to wait for an event, go to that event, and then download it as a mystery gift. However, the event to get Shaymin in America for Diamond and Pearl has already passed, so it is impossible to get it as a mystery gift now. There has already been a Pokemon Event for Shaymin (what you need Oak's Letter for) for Pokemon Platinum, so that has passed, too.
There was a special mystery gift that gave you the flute and you go into the place you found dialga.
You have to get the Azure flute through a Mystery Gift. The Event already passed though ):
you need the azure flute i think you get it from mystery gift then go to a Nintendo event
Either by migrating (you have to have a gba Pokemon game with a mew) or you can have someone trade you one.