To log in to phpMyAdmin you must first create a MySQL user and database. You can do this at the "MySQL Management" page in DirectAdmin. Use the details of the MySQL user to login to phpMyAdmin.
You can find phpMyAdmin at: http://yourdomain.uk/phpmyadmin, if this doesn't work, you can also access phpMyAdmin via your DirectAdmin control panel. The link to phpMyAdmin in DirectAdmin is available under 'Advanced Features'.