In the Finder go to the /usr/local folder (hit command-shift-G while in the Finder, and then type in "/usr/local" without the quotes). You should see at least two mysql folders (they may be named by their version numbers), and one alias named simply "mysql". You then need to change that alias to point to the mysql install directory you want to run.
If the mysql install you are looking for is not in /usr/local, it was probably installed in /Library/MySQL, where some of the OS X specific packages may install it. In that case, the alias in /usr/local should point to that mysql directory.
The alias in the /usr/local directory must be named mysql, since the system looks for a folder with that specific name to run it. Also, instead of using an alias, you can simply rename the directory with the proper install you want to use to "mysql", given that the directory is in /usr/local, and there are no aliases with the same name.