I need detailed instructions on how to get Darkri in pokemon platinum! Plz help me!!!

Have you obtained key items 'Member Card'?
You can obtain it through Wi-Fi event por downloading the member card. But the event has ended though :(
You must have that item to unlock the locked house at the Canalave City. With the member card, you can enter and the owner of the house will lead you to sleep in the bed. While you're sleeping, you'll instantly be moved to the Newmoon Island. Darkrai is ready to battle with you. His level is around 50.
This explanation is what I did after obtaining the member card, and I got Timid Darkrai with Dusk Ball. I suggest to capture Darkrai at the night so Dusk Ball will work best. And don't forget that he's so hard to be captured. SERIOUSLY >:(

Cheat, cheat, cheat, cheat, and cheat!
Use an Action Replay and the Wild Pokemon Modifier.
Or Walk through walls, and reach Newmoon Island, then use Sing (Thunder wave works also), False Swipe (Or get it down to 1-10 HP normally), then use an Ultra Ball. If it's still early on in the battle, Quick ball, or at night, Dark ball.