I was able to get my initial registration in using the Wii browser, so if you can't get registered for rewards in general yet, you could maybe try that, might get lucky. But, after that, you still need to register for the specific challenge, which is the tricky part. If you can get as far as registering for rewards in general, I would recommend going to register again, as that will log you in with the rewards account, but not filling out any more forms, but going straight back to xbox.com/rewards, and hope that you are still logged in. Then, hopefully, you can click on the old spice challenge and accept.