Do you have X11 software running, such as XQuartz? We have not tested Ferret on Mac OS XSierra, but I believe others have successfully installed it.
I confirm :-)
Note that for PyFerret, displayed plots on Mac OS XSierrado not work. Undisplayed plots (using the command-line option "-nodisplay") work fine. This appears to be an issue with the python library Qt4 or the version of the QT library that is used for graphics, under the new OS; we have plans to work on this.