DANGER - PowerPoint 2016 for Mac spell checker does not change custom dictionary to Add words!!!
The issue reported here was fixed:
PowerPoint 2016 for Mac spell checker does not Add words to custom dictionary
But further to that, I have noticed that when "Tools - Spelling" to Add words, PowerPoint 2016 version 15.31.0 (170216) does not automatically select the correct custom dictionary in "Add words to". Word does it right, but PowerPoint fails as said.
This is particularly dangerous, since you may add words to the wrong custom dictionary, for instance when you work with English and Spanish ones in the same document.