I don't think you need a Changes URL. It says in the prefs that this was "Optional advanced stuff", and I don't have a URL there in my prefs neither. (Except for the icon and my favorite weblogs, the whole "Optional advanced stuff" section is empty, and it's working all right.)

I think http://www.weblogs.com/ browses all the submitted pages once every hour and compares them to what they looked like 60 minutes earlier. If there are changes, the site appears on the Updates list, if not, it doesn't.

Sounds to me like you can submit any site to Weblogs.com, no matter whether it's a Manila site, Conversant site, any other weblog or just any web page.