If you can’t log into your mySQL with your admin username and password on a Plesk (> 11) machine, this is for you. Try:
# mysql -u admin -p`cat /etc/psa/.psa.shadow`
This takes the contents of the /etc/psa/.psa.shadow file and uses it as the password for your mySQL-server. This works for me and seems to be the new default. 😉
Hope this works for you, too.