How to Get Darkrai

1. Go to the little green house in Port Decca at night (Picture Shown Below) and after the short cutscene you will be taken to an island.

Once you’re at the island, you need to find the small area in the forest that takes you to the next area where Darkrai will be located. After you enter, click on Darkrai and you are now able to catch him.
Here is a video that helps!