For future reference, no registration is required, and there's no such thing as being "rejected" from a challenge. The most common problem that keeps people out of the challenge standings is that they download work before the challenge starts, either intentionally or because of their cache settings. In your case, I can see that you downloaded one GFN20 task about 45 minutes before the challenge started, and another about 10 minutes after it started. The first one won't count (even though you finished it during the challenge), but the second one does (but it didn't show up immediately in the stats because they're only generated every 15 minutes).