Private Events in iCal and Zimbra

Hi,
I use iCal in 10.6.2 and Zimbra server at work. I don't see an option in iCal to make events private, though I have heard that there should be a button on the edit pane to enable that. If I create the event on the Zimbra web interface I CAN get a private event to work - the problem is when I create the event via iCal - there does not seem to be an option there. Any ideas? Is it a config on the ZImbra side? OS X Terminal command?
Thanks