Darkrai

You cannot get Darkrai until after a Pokemon Event comes around, or you use an Action Replay to get the key-item called the Membership pass. When you get the Member-ship pass, go to Canalave City, and go to the Hotel, which is the building with the locked door, and the rest will be easy to figure out. After killing Darkrai, leave the Hotel, and go talk to the Sailor on the west side of the town, tell him to take you to Darkrai's island so that you can catch him, when you get to the Island follow the same path, and enter into the forest area, this time when you see Darkrai, catch him.

Well you need an action replay.. Then go to full moon island(cresselia's domain) adn make sure you have the walk throu walls cheat on... Then walk east as much as possible.. Ull eventually figure it out...

Well, you need to obtain the "Member's Pass" from a Nintendo Event (which are currently held in Japan and won't be held in the United States for a while) or by cheating. Then, use the Member's Pass to access the hotel north of the Pokemon Center in Canalave City. Take a nap in the bed, then when you wake up, you should see an item. Pick up that item and talk to Sailor Eldrich (in town) and have him bring you to Newmoon Island. In this island, there should be a tiny forest area with a level 40 Darkrai in the middle.

The way to get Darkrai is rather simple, hack, or when the movie The Rise of Darkrai comes out there will be a nintendo event in which they will give out Darkrai's.