It's very odd that they don't make it more explicit in the "What's New" window that pops after after an update. So once you have updated to 15.18 (released on 12 Jan 2016) you can find out how to use the new mac Selection Pane by reading this article that I just put together after having tested it:
http://youpresent.co.uk/powerpoint-s...-back-for-mac/