What to do with the member pass?

The Membership Pass grants access to Darkrai.

After you get the pass, go to Canalave City and head up. You should see this inn. Before, if you didn't have the pass, you wouldn't be allowed to go inside but with it, you can. Inside the inn should be a bed. Sleep on it and your character will wake up on New Moon Island. Follow the path and you will be led to Darkrai.