For a challenge to be associated to a quiz, you need to [one-time] add a QUIZ_COMPLETED action in Nitro Studio, which will fire when quizzes are submitted. Then when you go to make your challenge ("mission") rule, it should go like this:
Number of calls: >= 1 (or whatever you want)
Advanced / metadata rule: quiz_id = [id] (you can get the ID from the quiz URL in studio)
Advanced / metadata rule: pass = true (if you want to tie the challenge completion to passing only)
Complete docs and tutorial are here: https://docs.bunchball.com/#Widgets/Quiz.htm
Note: we have near-term plans to improve the ease of setting up a quiz challenge in Studio, as the above steps are a bit too technical in our beta version. but this should work for you.