I've found this method slightly more fiddly than the Use a 3rd party formatting tool method, but if you want to press on with the native Mac OSX way, here goes:
Are you sure you want to partition the disk “General UDisk Media”?
”.Once the format is complete, copy your “THEC64-drive8.d64” file across and try it on your c64 mini.
If problems persist, let us know in either the community forum or the facebook group.