I couldn't get anything from this link http://kb.dantz.com/display/2/index.asp?c=&cpc=&cid=&cat=&catURL=&r=0.3722345, but I've followed all of the steps that I've found specified and I'm having the same problem on a Server 2003 R2 (fully patched) with Exchange 2003 Enterprise using Retrospect 7.6.111.
I've set up a RBU id and done everything I can think of, Why can't I see the individual Mailboxes?