If you need to access the client MySQL instance on your Plesk 10.x box, you will need to connect to the admin instance first. To do that, do the following:
The password for connecting to the plesk admin instance is the same as your plesk admin password. If you need a refresher, run the following:
Okay, so once you've connected to the admin instance, you will want to execute the following:
The results will give you not only the admin username and password for the client MySQL instance, but also MSSQL if you have it installed.