You have to have the national dex.
it is in the upper right corner of canalave city
You have to get the member card and when you do, you lay in the bed and catch Darkrai.
at the very top on the right side of the river
You'll open it later on. I don't know when, but there's no key. I got this from a friend. Mabye you have to beat Champion Cynthia..... Thanks for editing while i wrote a whole paragraph and you post...