I just bought the Apple adapter on the basis of this thread, and the iMac is not seeing the BMD mini-monitor,
on Mac OS 10.13.3 and using desktop video 10.9.11 - the latest version approved by Avid for MC and Pro Tools.
SOLVED:
the BMD Kext had been blocked by Mac OS. Uninstalling and then re-installing the drivers gave no error message, but checking the system preferences:security and privacy pane, I could see that installation had been blocked and had to manually override the OS to install it anyway.
All working fine now - posting this in case it helps others.
Nigel