NI-VISA not working on Mac OS X

I have been using NI-VISA and Python/PyVISA on a Mac for instrument control. Now I've got a new Mac with latest os (10.11.6) and things are not working for me anymore. When I run my scripts I got this error message saying

VisaIOError: VI_ERROR_LIBRARY_NFOUND (-1073807202): A code library required by VISA could not be located or loaded.

I wonder if the GPIB/USB converter has the correct driver installed. In NI MAX I can see the device but vendor/model are shown as "unknown" which is suspecious. Is there a way to check if the driver is working properly?Also, first I installed my older version of NI-VISA (v15 or something) and since it didn't work, I installed the latest V16.0. I wonder if the new version over-write the old one or I have 2 versions co-exist. If anything I would like to uninstall all of them and re-install the new software, anyone knows how to uninstall NI software on a Mac?