OK, I upgraded to High Sierra, and installed the latest Desktop video. First attempt did not install the driver, I got no message that I needed to approve installation (except for my system password). Second time, I went to Security and Privacy settings before I restarted, there was a message if I wanted to approve Blackmagic to install the driver, clicked OK, and it works fine!
That message is hidden deep in the system prefs, and Desktop video should really display it on installation, nothing in the manual either on installation.
You can also see if the driver has been installed or not in the About my Mac-System report-PCI-then click the card and it will say Yes or No on Driver Installed.
When I mailed Support about it they basiclly said they don't support legacy hardware, and told me to ask in the forums... of course they know of this issue, how hard can it be to be helpful...