Did you enable maildir support in ISPConfig under management > server > settings on the mail tab? Have you sent a email to the account? The Maildirs where created when the first email arrives.
Please check your /var/log/mail.log for error messages and post them here.
Where should I start from in trying to figure out what's the matter? Thanks in advance for your help. And please pardon me for the cross posting; I put this on Ubuntu forums (http://ubuntuforums.org/showthread.php?t=421688), but no one has replied.
If you have problems with either ISPConfig or one of the howtoforge setups, It is always better to post the questions here as most of the howto authors and ISPConfig developers are not in the ubuntu forums