You have to add the *_enable lines to /etc/rc.conf, and then use /usr/local/etc/rc.d/mysql-server start to fire it up. See the comments in the rc.d/ file for the variables you need to set in rc.conf.
This is documented in the
rc(8),
ports(7), and
rc.conf(5) man pages, as well as the ports chapters in the Handbook.
Note: for the best performance on FreeBSD, use the 5.0.x series of MySQL.