On Windows machines there are two sets of such files, one in the Program Data section and one in the individual user's %appdata% folder. I suspect Macs use a similar system, a default section plus a "copy" made to each individual user's section of the OS so that customizations can be different for different users of the software on a given machine. I suspect that you added your custom style to the general section used for the original installation of the software rather than to your personal version of the defaults.

SImon I am on a Windows machine and I too think the way Finale handles these things is goofy but I can get it to work.

I think the easiest way is to just make a 'default' musx file and leave it in you general music folder. Where you save any Finale file.

Just make a basic file that has all the settings you want and open it each time.  Do a quick save-as and notate away. You can add or delete instruments as you like. Add measures, whatever.

Make sure you follow the path shown in Preferences > Folders. As Adrian said, allowing for platform differences, it has to go in your user library, not in the general one!

Note the different roots here: Plug-ins are in the hard drive library, while document styles (and all the others) are in my user files.

i'm not on Big Sur but in Catalina, I had to navigate manually after the first time I placed a style sheet in. Subsequently, it showed up as it should.

Thanks for all the responses.  I think I've found a way forward, so much appreciated -- can get back to writing music rather than banging my head against a wall.

Cheers all,

Simon