The web add-in still installs fine on Outlook for Mac, it's just that there is a discrepancy between the behavior I am seeing on Office 365 Outlook and Outlook for Mac (this is also true when testing on Outlook for Windows, which has the same behavior as the Mac version). I have a feature for a Microsoft Outlook web add-in which works fine in Office 365 Outlook, but when I test the same feature on Outlook for Mac or Outlook for Windows, the same feature doesn't work.