If you don’t want to install MySQL via MacPorts for whatever reason and want a clean installation to work with Ruby on Rails and the mysql gem, you need to do this: Go to http://dev.mysql.com/downloads/ to get the latest version of MySQL (by clicking on Download for Community Server). Download the Mac OS X package format and choose Mac OS X 10.5 (x86). Important here is to choose x86, not x86_64.