in the control panel go to Domains==>Manage Domains and click on the link to add a new domain/subdomain. From there you just type in the domain name and click fully host this domain (i don't mess with any of the stuff checked there so unless you want to use php5 or to have fast-cgi support you don't need to worry about that...)
From there, all you need to do is log into your account at whichever registrar you are using and edit the name servers of the domain to ns1.dreamhost.com, ns2.dreamhost.com, and ns3.dreamhost.com (if they allow you to list a third server).
The Insane Cabbit
My Blog http://www.sounanda.com
My web store:
Send me a pm if you want cms or forum software installed (for a fee)