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