Hey Simon, fret not!
First, there's the instructions from MySQL:
Second, if you can't get that running properly there is an article at Apple on building from source (although that makes keeping the system current a little more difficult for non-unix types):
Even harder than that, will be the magic, top-secret voodoo required to get ruby-mysql installed on OS X, to which I have yet to accomplish.
Being a PostgreSQL homer means I just did everything in PostgreSQL, but I will eventually need to have access to MySQL from Ruby. Anyone else out there know the trick to get ruby-mysql to build with the stock MySQL dmg provided by MySQL.com?