A number of users have reported attempting to open Rhino 5 or Rhino 6 version 3dm files in Rhino 7 for Mac results in a hang - often with a blank dialog that looks something like this:
minus the 
emoji of course…thought that may be what you are feeling.
We believe we have fixed this issue in the latest update to Rhino 7 for Mac 7.3. If you are experiencing this issue, please Quit and relaunch Rhino, then update to 7.3.
Sorry about this…it’s not a good first impression.