I was having some trouble getting this to work. It seems like the package in MacPorts has changed many of the default locations and these need to be specified explicitly at present.
$ sudo port install mysql55 $ gem install do_mysql -- --with-mysql-dir=/opt/local/lib/mysql55/ --with-mysql-lib=/opt/local/lib/mysql55/mysql --with-mysql-include=/opt/local/include/mysql55/mysql/
I guess that this will on be useful for a short time, until
do_mysql updates it's default search paths.