I had same problem setting up a connection from Ical and I solved following suggestion from Zimbra wiki:
CalDAV with Leopard iCal - Zimbra :: Wiki
The wiki gave not clear info on some points, let me try to describe my steps:
1. On Leopard (10.5), open iCal.
2. From the Menubar, choose "iCal"(Next to the Apple menu)
3. Choose "Preferences"
4. Choose "Accounts"
5. Click the + (plus sign) to add an acocunt
6. Give the account a Description
(I used "Office")
7. Enter your Zimbra username
(for me firstname.lastname NO email address)
8. Enter your Zimbra password (
)
9. Click the arrow to expand server settings
10. Enter the Account URL USING following syntax http://"zimbraserverdomainname"/principals/users/Zimbrausername/
(in my company I used http://zimbra.xyz.com/principals/use...name.lastname/ )
11. Click Add
It works!
P.S. please verify correct settings (if you have it) of the iSync Zimbra prefpane)