you can login to mysql under 'admin' account by typing in root console
mysql -uadmin -p`cat /etc/psa/.psa.shadow` mysql
admin has all privileges.
under this account you can create additional administrative accounts, check mysql docs for it.
Also check DB servers setup page in Plesk settings. It is possible to use phpMyAdmin to add new users with administrative privileges.
And finally. Mysql 'admin' user has the same password as Plesk 'admin' user. And if you change password for Plesk 'admin', password of Mysql 'admin' will be changed to the same automatically.
Also. There is no 'root' user in the Plesk instance of Mysql. Only 'admin' is used. You can create root as in the traditional instance.