Just a follow-up to my problem above:
Langmuir Support helped me get the .dll file installed by sending me a compressed version. I opened it up and was able to click and drag it into the Mach3 folder without any problems. This worked great and now Mach 3 has the needed motion control file. Thanks Langmuir Support (Zachary). Not sure why my downloaded version wouldn’t open or save, but chalk it up to Windows 11 security preventing me from saving a .dll file. Oh well, it works now!