How to import opml file (mind map) from Mac

I am teaching Scrivener, the majority have windows, but 2 have Mac, and I dont know how to import opml file with extencion .mm

That isn’t a feature on the Mac, it just supports OPML. If you have a way of converting the .mm files to .opml then once you do so, simply drag and drop the .opml file into the project binder.

On Windows, use the File/Import/OPML or Mindmap File menu command for .mm files.