I was running logistics for a gototraining webinar and after I made someone else the presenter, they gave attendees the ability to use their pens (to draw on the screen for an activity), and the attendees (and myself) couldn't see the drawings, only the presenter could. She was using this operating system: Mac OS X El Capitan Version 10.11.4 (iMac Late 2014). And this browser: Firefox 46.0.1. And this version of PPT: For Mac 2011, Version 14.6.4. Any ideas about how we can get the attendees to see their drawings?
I tried duplicating the scenario internally with one of our support agents just as you described using the same Mac OS and software versions. Anyone that was joined into the session was able to see the drawings being done from either side, regardless of who was drawing. We could not find any other reports of anything similar from other customers either, so it may be something isolated either to that session or that one presenter possibly.
At this point, the best way to further diagnose what might be happening would be with a call into our support so that an agent and get eyes on the scenario and troubleshoot live. If drawings were working fine for the other attendees until this one presenter took over, having that person on the line as well would be very beneficial.