If the panel (not cPanel, as that's some other software), seems broken, try a different browser. Or check status, as already posted.
With Filezilla, you'll connect to a user listed in your Manage Users section. The domain doesn't matter much as long as it's one you've set up here. They're all on the same server. You can also click on Account Status in the upper right of the panel to get your web server name, then connect to SERVER.dreamhost.com
A backup method to contact support is here: