You have to set up a mirrored domain. Go into the panel > Manage Domains > add a new domain. Use the box for a mirrored domain, and use something like yourdomain.dreamhosters.com, and select your domain from the drop down menu.
This will set it up so that in a little bit (maybe a few hours) you can go to youDomain.dreamhosters.com and see the content you have up-loaded to your new DH account.
There's also a sigguestion (Home > Sigguestions) so that Dreamhost would have automatically set this up for you:
Automatically set up domain.com.dreamhost.com for new domains to test before DNS propagates.
Good luck (and a search would have reveled this information)