if you're using a ftp client it's probaly just domain.com or possibly ftp.domain.com (repalce domain.com with your domain name)
If you DNS hasn't propiagted yet (as in you if you go to www.yourDomain.com and it says unknown) then you should have gotten an E-mail from DH when you siged up instructing you how to hook up with FTP before DNS propiagated. You'll enter in something to the effect of machine.dreamhost.com
The user name and pass will be the one that you set up with the DH panel. Not necessarly your user/pass for the panel, but you had to set up a user in the panel when you signed up. That'll be the one.