After installing Homebrew, Git and MySQL, add the MySQL Gem via gem install mysql2.
Now you should be able to create new rails apps set up to work with your mysql-instance via rails new yourappname -d mysql or edit your database.yml according to this:
development: adapter: mysql2 encoding: utf8 reconnect: false database: yourappname_development pool: 5 username: root password: socket: /tmp/mysql.sock ...