For DV servers, Plesk renames the root account for MySQL to admin. Whenever referring to any documentation regarding MySQL's root user, just consider this user admin on your server.
To access root privileges to MySQL, simply login with the admin username instead. The password is a hashed version of the admin password used to login to the Plesk control panel. This can be found in /etc/psa/.psa.shadow
If you need to change your admin password, you can do this in your AccountCenter. See this article for instructions: How can I change my Plesk admin password?