While logged in, go to http://www.livejournal.com/modify.bml and click "Proceed". On the next page that comes up, scroll down until you see "Domain Aliasing." Enter your domain name in the text box.Modify.bml has since changed and the FAQ should be updated to say something more like:
While logged in, go to http://www.livejournal.com/modify.bml and scroll down until you see "Domain Aliasing." Enter your domain name in the text box.
*jack