Just ask a TM at the beginning of the queue.
We asked three times over two days - the first time there wasn't anyone available to do it, the second time they said it would take 30 minutes for someone to get there (so we declined), and the third time they asked us to wait right inside the doors (by the metal detectors) and someone was there in about 10 minutes. Try to hit a time that isn't too busy.
Isn't the greatest tour, but it was pretty fun. :thumbsup: