The domain that you wanted to add on "manage domains" is the domain that will be hosted eventually.
Since DNS for that domain currently points elsewhere, there is no path to what you just added. One way of doing that is to add a mirror, that could either be a domain or subdomain that you own that dreamhost also controls the DNS for, or it could be a free sub-domain that you created using the free dreamhosters.com doman.
HOWEVER, pay close attention to that warning about CMS's. Consider choosing one of the other methods on the page, specifially either edit your hosts file locally or use dreamhost nameservers for your local machine. The simplest for your case is to use the hosts file method.