Moving thread to Known Issues, reposting a Best Answer since this seems to be common:
  If you are on a Mac and experiencing the issue where libraries do not install, try the following:   1) Close Vectorworks. 2) In your Application folder, right click on the Vectorworks 2017 folder and choose "Get Info"
3) Set the permissions so that each listed user (System, YourUsername, etc) is allowed to Read and Write. 4) Then, click the lock at the bottom right, unlock it with your admin password if prompted 5) Click the gear and choose "Apply to enclosed items" like so, it may take a few moments to finish:     6) Reboot your machine, then restart Vectorworks and try installing the libraries.