The Office Timeline Add-in does not support PowerPoint for Mac.
Office Timeline cannot be installed on Mac OS’s and does not work with PowerPoint for the Mac. It can be deployed on a Mac virtual machine using desktop virtualization solutions like Parallels or Bootcamp to run Windows; however, this is not a tested scenario.
Mac users can use the online version of Office Timeline by going to https://online.officetimeline.com/
See Windows PC System Requirements for the PowerPoint add-in here: http://support.officetimeline.com/hc/en-us/articles/201950763-System-requirements-for-Office-Timeline