This was the "full" version of Retrospect on the Mac. It sounds like Retrospect Express also supports Internet backup sets, so that's not the problem. One thing to try is whether you can manually ftp a file to the directory you're using for backups.
I did have one issue, which is that Retrospect only supports active mode FTP. This can be a problem if the machine doing the backup is located behind a firewall. (Active mode FTP requires that the FTP server make a connection back to the machine that started the FTP, which isn't possible if you have a firewall set to block incoming connections.)
It turns out that some firewalls try to handle this. I have a Linksys BEFSX41 and while it worked most of the time, it would fail randomly. I thought maybe DH was limiting FTP uploads or something, but it turned out to be a bug in the Linksys firmware.
The solution was to download an open source FTP proxy called Frox, which can do active-passive conversion. After that everything has worked fine. Every month I rotate backup sets, and I backup ~800MB of data overnight, which takes several hours over my basic DSL connection. During the month I do incremental backups which are pretty fast.