You can either:
Go to a special event
Use mystery GIFT
Use action replay
Inside you go to sleep and end up on an island with darkrai on it
You can use wi-fi to get the key to the door by sometime in September right now
i have no ideal
either trade from diamond/pearl or use action replay to get the member ship card and go to the house that was locked in canalave
Get event on action replay.
get the member pass from a Nintendo event and then go to canalave city to that locked door
in locked house in canalave city
He is the house by the pokecenter in canalave city
you have to go to canalave city and go to the house which is usually locked
togekiss learns sky attack by tutoring.
go to canalave city and go into the house on the right hand side and in the middle there is a house go into the house and talk to the man
in an event, you have to get the member's pass then go in the locked harbor at the top of canalave city above the pokemon center.
Get the azure flute from the locked house in canalave city. only on diamond or pearl though.
to catch Darkrai you should have the key item that allows you to enter a locked house at Canalave City... then you will be put into sleep waking at full moon island.. save and face darkrai...