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:
If you are connecting to a share called "Office Calendar":
Interesting reading on modifying share mount points and permissions from the CLI:
Let us know if you have any more questions.