[CdtDelta]

Ok well I updated to 5.0.2 and tried it again, still no joy. I am using the Open Source version if that makes a difference.
Is there something I need to enable on the Zimbra server in order to use CalDav? Or is it enabled by default?

Just looked at the logs on my Mac, and it's showing this when I try to connect:

Mar 18 13:20:16 Slave-1 iCal[597]: [DAVRequest _readStreamEvent]: SecTrustEvaluate failed. Failing with error: Error Domain=NSStreamSocketSSLErrorDomain Code=-9813 "Operation could not be completed. (NSStreamSocketSSLErrorDomain error -9813.)"
Mar 18 13:20:18 Slave-1 iCal[597]: *** -[NSLock dealloc]: lock (<NSLock: 0x16a19e00> '(null)') deallocated while still in use
Mar 18 13:20:18 Slave-1 iCal[597]: *** Break on _NSLockError() to debug.