You can use shell access (telnet in) to edit the permissions if you know some simple Unix/Linux commands, or if you are using WinSCP to Secure FTP your files in, it allows you to change that right there in the FTP client.
You should probably read this on permissions:
(part of the Dreamhost Knowledge Database)
and do a little more looking around the help area mentioned above. Changing permissions can royally f*ck up your site if you don’t know what you’re doing.
Worst comes to worst, email support and ask for advice on how to approach this. I’d think they’d rather can that email than one that says, “Sorry I messed up my entire account.”