If you want to really benefit from customizing your Ribbon in PowerPoint 2016 for Mac, then certainly add some commands that are not placed by default within any of the Ribbon tabs. You’ll notice that all commands are placed within Groups. Whatever your intent may be, you cannot place any commands within the existing Groups that are built within PowerPoint. You first need to add a custom Group within any of the tabs available in the Ribbon. Thereafter, you need to populate them with commands.