Hi Drew,
To continue with your current trajectory, I would suggest downloading the official binary installer for MySQL (MySQL :: Download MySQL Community Server). 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.
Best,
-r