Can you set the nameservers for your domain? If so, just set the nameservers to ns1.dreamhost.com, ns2.dreamhost.com, and ns3.dreamhost.com
Then all you need to do in your DreamHost control panel is add a new domain and then take it from there.
If you can't set the nameservers, you will need to first add a new domain to the control panel as above. After you do so, you will be able to click on a "DNS" link on that domain and see what the IP address is of the server you're hosted on. Then you'll need to add both a blank "A" record and a www "A" record at freedns.
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