If this is a new install
If you did not choose the server variant you may add a mysql user and you will need to change the ownership to that user:

sudo chown mysql /opt/local/var/db/mysql
sudo chown mysql /opt/local/var/run/mysqld
sudo chown mysql /opt/local/var/log/mysql

You might want to run

sudo -u mysql mysql_install_db

You have to symlink mysql to the location on osx

first find out where mysql has put the socket:

mysql_config --socket

Next symlink /tmp/mysql.sock to the right location of the socket

sudo ln -s path-of-mysql.sock-from-above /tmp/mysql.sock

Start MySQL

sudo /opt/local/share/mysql/mysql.server start

Stop MySQL

sudo /opt/local/share/mysql/mysql.server stop

A startup item has been generated that will aid in starting mysql5 with launchd. It is disabled by default. Execute the following command to start it, and to cause it to launch at startup:

sudo launchctl load -w /Library/LaunchDaemons/org.macports.mysql5.plist

Installs a package

port install package

uninstalls a package

port uninstall package

shows all ports currently installed

port installed

shows all ports not currently installed, you may want to pipe the output of this command to grep to limit the output

port uninstalled

search for a port whose name matches the regex

port search regex

shows meta information on a package

port info package

shows the latest version of all available ports

port list