Someone here just pointed out to me that that’s not our message, that’s Google’s message. You can read about it more here:
IE11 is supported (see Browser Support | Maps JavaScript API | Google Developers), but as the Microsoft article points out, it’s probably something to do with misreading the userAgent. If there’s any way you could hit ctrl-l in the add location window, and go to http://www.whatsmyua.com/ (what’s my userAgent), and past it, that’d be helpful.