Per Cyber.mil, STIG Viewer 3 is not and will not be available for Macbooks, which causes me significant issues as that's my primary workstation. To work around this, I created a STIG Viewer for Mac that runs from the CLI. You simply download the latest STIG zip file to the repository, and run the STIGViewer python script. As the script does not uncompress the file, there is a bit of a hit to the memory, but if you are looking for a tool for Mac, this would work for you.