To continue with your current trajectory, I would suggest downloading
the official binary installer for MySQL
(http://dev.mysql.com/downloads/mysql/5.0.html). However, if you were
willing to use sqlite and are using mac os 10.4, it's already installed
- just install the driver (gem install sqlite3-ruby).
However (and blowing my own horn here), if you truly want a rails
sandbox for os x, just download Locomotive
(http://locomotive.raaum.org) - there are quick start directions in the
Locomotive application help.