Taken straight from <a href="http://www.eclips3media.com/workshop/2008/10/installing-the-mysql-gem-on-os-x/">this blog</a>

in a terminal.. First do a locate mysql_config and then replace the path in the following command with where that file is.

$ sudo gem install mysql -- --with-mysql-config=/usr/local/mysql/bin/mysql_config 

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Building native extensions.  This could take a while...
Successfully installed mysql-2.7
1 gem installed