Is your OS 10.7 or newer? Is this newly downloaded from Creative Cloud? Is it licensed or still a trial? If you click on Library does anything happen? If you use Shift-Tab will the side panels return? If you use F or Shift-F does the top menu return?
Yes it is newly downloaded form Creative Cloud as a trial. I am up to date with OS 10.9.4. Shift+Tab doesn't do anything for me, and the F and Shift+F acts like it's changing screen modes, but the actual look never changes.
Tab is the Mac shortcut for the side panels.
I would suggest closing Lightroom, rebooting the Mac and if the same result, delete Lightroom and redownload.