In the DreamHost panel in "Domains>Manage Domains", find the subdomain you created and click on the "DNS" link to get a page that tells you the IP address of the subdomain.
On the other host, add an "A" record for "wiki" to the DNS for xxxx.com. If you want, you could also add a "www.wiki" entry.
BTW, this is what I've done for a domain of mine in order to take advantage of the one-click installs for mediawiki here at dreamhost.
Finally, for cleanness, don't forget to delete hosting for xxx.com on DreamHost. Don't delete the domain, just delete the hosting. Let me know if you can't find the button to do this.
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