Hi there,
after re-checking everything I realized I had overlooked your initial instructions to enable the plugin managesieve and autologin in config/main.inc.php. After doing that I got the Filter tab. Halleluja.
Unfortunately I could not send any emails using the webmail. And after adding a test filter to my it account I can no longer recieve any email to it.
I solved the sending email through rouncube by:
removing '%u' and just leaving '' in SMTP user settings in .../webmail/config/main.inc.php
Still trying to figure out why one of my accounts now can't connect to read IMAP mails anymore. (Sending works, "Connection Doctor" in mail works, but no emails, even after re-creating user in 10.8 Server.app and from WGM).
Anyways, getting closer, thanks so much for the help so far and I sure hope Apple will figure out a way to make this implementation easier in the next release. (Considering they provide scripts to configure Promise raid's, why not provide scripts to configure a users own download of roundcube.. right? ;-)
/Simon