When I've opened multiple mindmaps, the only way to switch between them is via the "Window" menu. In previous versions I always used the "tabs-functionality" which makes switching between maps much easier. I can't find this functionality. Is this still part of the software?
Could it be that you have been using MindManager on Windows before? Current versions of MindManager for Windows still have a tab-based interface, but the Mac version has never (I think) worked with tabs?