Just add it as "fully hosted" in the dreamhost panel. If you purchased it from a different registrar than dreamhost make sure the nameservers are correctly set to ns1.dreamhost.com, ns2.dreamhost.com, ns3.dreamhost.com.
Don't forget DNS propagation. When you make changes setting things up depending in whats cached where, it might take 4-8 hours (longer sometimes) for DNS to propagate.
If you're still having problems we need more details to help.