We goofed a bit earlier and asked you to opt in for the challenge matches before that option was even available in your profiles :). Ooops!
In any case, it’s there now. If you plan on participating in one of the sponsor challenges please edit your team profile and select your challenge from the list.
You can opt in any time before the contest build weekend ends, but we encourage you to do it early so you don’t forget. Also, if you’re still thinking about what you want to build, there are some great suggestions there – everything from Twitter apps to A/V ideas to SAASy goodness. So don’t freak out, yo.