Mac OS X 10.8 no longer includes X-11 as an option. If you need to run software that requires X-11, you have to install XQuartz 2.7.2 or later.
You can download XQuartz 2.7.2 at http://xquartz.macosforge.org/trac/wiki/X112.7.2
[kirkmc adds: I haven't tested this. I haven't needed X11 for donkeys' years.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20120719101352282