Try to use existing mysql under OSX.
$ sudo gem install mysql -- --with-mysql-config=/usr/local/mysql/bin/mysql_config
Go to mysql.com, download and install the 64-bit OSX 5.1 .DMG installer.
$ sudo env ARCHFLAGS="-arch x86_64" gem install -V mysql -- --with-mysql-config=/usr/local/mysql/bin/mysql_config
With MacPorts, install mysql5 and mysql5-devel.
$ sudo gem install -V mysql -- --with-mysql-config=/opt/local/bin/mysql_config5
$ irb
>> require 'rubygems'
=> false
>> require 'mysql_api'
=> true
>> Mysql.get_client_info
=> "5.1.34"
>> Mysql.get_client_version
=> 50134