I downloaded the binary from here and it works on my system (OSX El Capitan). Mix is under hard development as I understand so a thing might be working today, but broken tomorrow. Just have a bit of patience (I know it's frustrating ;) ).
If that doesn't work for you, maybe you could also try homebrew. Haven't tried it myself but it should be as easy as:
brew tap ethereum/ethereum
brew install cpp-ethereum
brew linkapps cpp-ethereum
brew install cpp-ethereum --with-gui
If none of these things work consider installing Sublime or Atom. I find they are more pleasant to work with but don't provide the debugging options that Mix has.