There's a new update to MGB, so please click the button 'check for updates.'

Michael changed the latest Mac version app name to "MoI v3.1.app" and MGB looks for the legacy name: "MoI v3.app"

I've now fixed it. You should press the check for update button on the top right of the interface, and it will automatically update the program. No need for Windows users to update, but if you do things should work just fine.