When I try to use any version of the Entity functions, the function is returned as if it were undefined. For example, EntityList["Planet"] returns EntityList[Planet]. The help examples don't work either. Is this happening to everyone, or just me?
I'm running V10.1 on OS X 10.10.2 (Yosemite). I ran into this problem about two weeks ago. I solved it by shutting down Mathematica, throwing the Paclets folder found in ~/Library/Mathematica/ into the Trash, and restarting Mathematica. After restarting, I tried the same "Planet" example as you did and it worked (after downloading some new paclets).
I have not had any trouble with
Entity expressions since.