1. start up a Terminal window (/Applications/Utilities/Terminal.app)
2. type in:
"sudo xattr -rd com.apple.quarantine "
without the quotes (and note the space at the end -- it has to be there)
3. Drag/Drop the uRTcmix folder that should be in your Unity project
"Assets" folder into
the Terminal window with the "sudo ..." command typed in. The result
should be a line in the Terminal window that looks something like this:
sudo xattr -rd com.apple.quarantine /Users/whoever/myUnityproject/Assets/uRTcmix
4. Hit <return>, give your password, and hopefully(!) all will be happy.
Unfortunately you will have to do this for every Unity project that
uses uRTcmix.