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:

• Run the “Disk Utility” tool in Mac OSX
• Select your USB stick item in the left-pane (and not any existing partition within)
• Click the “Partition” tab

• Open the “Partition Layout:” combobox and select “1 Partition”

• Change the “Format” field from the default of “Mac OS Extended (Journaled)” to “MS-DOS (FAT)”

• Then click the “Options…” button and change the default “GUID Partition Table” to “Master Boot Record”
• Then click “OK” button
• Followed by “Apply” button

• You will then get a warning popup “Are you sure you want to partition the disk “General UDisk Media”?”.
• Click the “Partition” button to accept this.

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.