With the admin account you need to configure a folder for calendars. Click "list calendars" and add a new calendar. Enter a name with spaces and select /homes/<username>/. Then in the Sunbird you need to enter:http://
notice "home" without the "s" in the URL.
CalDAV still isn't working right, though. The name of the calendar is always fixed to what you had setup with the admin account. Rename doesn't work. Adding new calendars in sunbird (or Apple iCal) to the newly created CalDAV calendar works, but the name is always "untitled" and can't be changed.