You need to create a Fully Hosted Domain in the panel.
DreamHost customers can piggyback onto the dreamhosters.com domain for free. Follow the same procedure of hosting "yourdomain.com" by going to the Control Panel, then Domains -> Manage Domains. At the bottom is a Create New Domain (or Sub-Domain). On the next page, use the top box for a fully hosted domain, calling it whatever.dreamhosters.com, owned by your user, then submit the form. You'll get a whatever.dreamhosters.com directory in your home directory, which you can fill, then wait for DNS to propagate.