Hey I've just got hosting through Dreamhost and was wondering what the URL is to access the site without a domain name...I was thinking username.dreamhost.com/etc but it hasn't worked.
There is no default, because there is no default site on a new account. But you can set up any number of http://$WHATEVER.dreamhosters.com through the panel, which will host a directory or symlink named ~/$WHATEVER.dreamhosters.com after a couple minutes.
If you're planning on buying a domain name:http://wiki.dreamhost.com/DNS_-_Viewing_site_before_DNS_change
Otherwise, yes, create a Fully Hosted Domain called YOURDOMAIN.dreamhosters.com