Your question is not very clear, as it is not obvious what you mean by "forwarded".
If you have registered a domain name with DreamHost as the registrar, and you want to point that domain to a site on another host, you will have to change the nameservers for that domain to those of the other host.
On the DreamHost control panel, is can be a bit "unintuitive" to tell where to do that.
Log into the DreamHost Control Panel, and go to the Domain -> Registrations screen. There you will see all the domains you have registered with DreamHost. "Check" the check box under the "Modify Whois" column (immediately to the right of the domain name) for the domain name you wish to modify, and click the "Modify Whois for selected" button at the bottom of the page.
On the resultant screen, scroll down till you see the DNS sections, and complete the section called "Set DNS info to host your domain elsewhere" by entering the nameserver information for the host housing the domain, and submit the form.
Presuming that the other host has an appropriate record for that domaion in their nameservers, once DNS has updated across the web, you will be good to go.
If, on the other hand, you only want to have someone browsing to the domain you registered at DreamHost "forwarded" (re-directed) to a different domain on a different host, you need to have a hosting plan at DreamHost so that you can "add the domain" to it and then either modify the DNS or setup your own re-direct.
DreamHost does not provide custom DNS records for "registration only" accounts; I don't know if support could help you with this (you could always ask)
This thread might help make some of that a little more clear If I've misunderstood your question, please post back with some additional details and I'll, or someone else, will try to get you the answer yo need.