Where does MySQL store its database files on OS X?
It should be stored in: /usr/local/mysql/data
When you use mysql installed via Homebrew, you should find your database folders and files in /usr/local/var/mysql
OS X Server (at least v10.6) keeps the database in /var/mysql by default. You can change this in Server Admin -> MySQL service -> Settings -> Database location (note that editing this field doesn't actually move the data, just changes where it's loaded from).