I just installed the birthday reminder zimlet on my 8.0 server. It shows up under Zimlets, and when I click it a dialog shows up. As it is labeled "Birthday Reminder: Preferences" I think it should be possible to set the options for reminder creation there. There aren't any options displayed, though, only the note about a new calendar being created, a horizontal line and two buttons labeled "Start Address Book Scan" and "Abbrechen" (cancel in German).
When I click the Start button nothing happens. Cancel closes the dialog, of course.
Does the zimlet not yet work with v8.0?