I've just released a new 3.1 version of MoI (Mac version), to fix problems when running in OS X 10.11 "El Capitan".

You can get MoI v3.1 by putting in your license key here: http://moi3d.com/reinstall.htm

This version should fix launching MoI on 10.11 and also fix problems with maximized viewports not appearing correctly.

Disabling rootless or installing XQuartz separately are not required (there was some previous discussion and testing for these).
If you've previously disabled rootless you can re-enable it again with the instructions here.

Please let me know if you still see any problems.

Thanks,
- Michael