The shared calendar code has been temporarily removed from the iSync connector until we squash bugs associated with it. As a workaround, you can use iCal's calendar subscription feature to connect to a shared calendar as read only. I believe you can mount a calendar read/write using the Mozilla's Sunbird plug-in for Thunderbird. Sorry for the inconvenience. We'll get that code back in there as soon as we can.
Connecting to a published Zimbra calendar with iCal or Sunbird may require server version 4.5 if you are requiring authentication on the share. We fixed a bug in 4.5 pertaining to this. To connect to the share: http://server.domain.com/home/grantor_userid/Calendar
If you are connecting to a share called "Office Calendar": http://server.domain..com/home/grant...ice%20Calendar
Interesting reading on modifying share mount points and permissions from the CLI: http://wiki.zimbra.com/index.php?title=Zmmailbox
Let us know if you have any more questions.