The current relaese of Rhino BETA, Version 6 (6.16.19149.14184, 2019-05-29) on Mac OS has problems opening/loading STL files. The previous Rhino version worked OK, this release shows a spinning icon and nothing happens.
We are aware of this one and are working on it: RH-52971.
Please correct me if I’m wrong, but you should be able to start a new document, then import or open the STL. Consider this a very-temporary workaround.
This should be fixed in the latest RhinoBETA for Rhino 6 for Mac. Please give it a try.