Is this a new install, or has this install of WordPress been running for a while.
Did you manually install, or use DreamHost's easy-to-use one-click method (in your control panel)?
If this install has been running for a while, your MySQl server is probably down. Wait it out or contact DreamHost support.
If this is a new, automatic install from your DreamHost control panel, contact DreamHost support.
If this is a manual install, you probably have one of the values (most likely DB_Host) wrong. "localhost" is not the correct value. Login to your DH control panel and go to Goodies/MySQL for all the information you need.