While I try to contain everything about installing MySQL in a single post that I update from time-to-time, Mac OS X, Mountain Lion (10.8.x), requires some pre-steps. You must install XCode, and the Command Line Tools. This post provides the screen shots and step-by-step instructions.
Before you can do any of these steps, you must connect to the Apple Store and download XCode. Dependent on your internet speed this may take some time. Generally, it’s an hour or less.