This looks like something that was mentioned before for the Game Help ( Could Be Wrong , Memory Is A Bit Fuzzy ).
Registration happens at random and it stays open for atleast an hour or two. From what's been said so far, the reason why the registration isn't constantly open is because so many people try to get in and that can mess up the server. ( Again could be wrong ). So even though there isn't a set time for when registration opens, all you can do is keep checking in and soon, you'll be able to make an account.
Hope this shed some light to your question ^^
Like Hakumi said, the registration opens randomly for a couple of hours (about 2) and then closes. There isn't a specific time it opens, though. It's just a matter of being patient. I know it may be annoying to 'ye, but it's the only way of preventing tons of people of getting in.
You'll catch it sometime. Just check every so often and refresh the page. (: