If you look through your support history on the ControlPanel you will find the original Joomla message with the access details for phpmyadmin. Or, you could add another user/password to the Joomla database to access phpmyadmin. Or, use another MySQL hostname to access.
Opinions are my own views, not DreamHosts'.
I am NOT a DreamHost employee OK!! :@
You act on my advice at your own risk!