I've had some limited success with the Zimbra Evolution connector and the Zimbra calendar.
Here's what I did:
Go to Calendar - Add New Calendar
Calendar Type: On the Web
Name: John Doe
Use SSL: Unchecked
The email account ("email@example.com" in the example above) must be a valid user on the Zimbra server.
I have found that the Evolution interface for calendars on the web is somewhat fragile. First, as good measure, I suggest restarting Evolution whenever you make changes to the calendar parameters. Only after a restart did it take my changes. Also, you may want to double check the parameters after you enter them. Evolution will sometimes mess them up.
I can see the calendar, but I cannot update it from Evolution. It was my understanding that the connector would allow a two way update, but I have not had much success with that.