I've just followed the instructions at https://docs.moodle.org/en/Moodle_Mobile_quiz_offline_attempts.
It's a bit complicated to set up, the download/sync process not so easy to understand, but I managed to get it going. Doing a quiz offline is IMHO rather frustrating, since the student does not get any feedback. Not useful at all for learning, only for testing, so really a very limited interest.
Joseph
