Unable to Use PhpMyAdmin in Cpanel
From Lunarpages Web Hosting Wiki
If you are logged into WHM with your Master Reseller username and password, and then try to access PHPMyAdmin via the client's cpanel you will receive the following error:
You will need to log into their Cpanel using the username and password you created for them.
Click the Logout icon on the top right of Cpanel, then type in the username and password.