Assuming you have the correct driver, it may be a new “feature” of Mac OS X which prevents loading unsigned drivers. Unfortunately the page with the really good instructions appears to be down at the moment, but I’ll give you the short version of what I did.
1) Open the Terminal and issue the command "csrutil status”. If the response is enabled, you need to disable it. If it is already disabled then there is another issue and the rest of this doesn’t apply.
2) Boot into recovery mode. To use Recovery Mode, reboot or start your Mac and hold the Command and R keys simultaneously on your keyboard as soon as you hear the familiar startup chime. Keep holding as your Mac boots, which may take a few moments depending on its specific configuration.
3) Open the Terminal again and issue the command "csrutil disable”.
4) Reboot to normal mode and reinstall the appropriate driver.
Hopefully this resolves the problem for you.
Cheers,
Jim
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Jim McCorison
Orcas Island, WA