There is no error in the howto. If you use ipop3d as pop3 daemon, the mailbox command shall look like this:
mailbox_command = procmail -a "$EXTENSION
Have you installed courier-pop and courier-imap as described later in the howto (end of page 5)? Then the mailbox command must be empty:
For courier you will also have to switch to maildir delivery in ISPConfig under Management > Server > Settings on the mail tab. Otherwise you mail gets delivered to mbox files and you cant fetch them with your mailclient.