When you buy a domain name registration thru dreamhost, you also have to choose how you want that domain hosted. (unless you changed them, nameservers will be set to dreamhost by default.)
If the nameservers on those alternate domains are dreamhost name servers, go the panel and click on “manage domains”, then click the add domain / sub-domain button at the top of the page. On the page that follows, scroll down, the page is made up of several independent “forms” the first of which is called “fully hosted”. In this case you will want to till out either the “Redirect” section or the “DNS Only” section of the page depending on the needs of the receiving site.