First make sure you can successfully obtain a kerberos ticket with
kinit and you can get to your AFS home from your Mac. If you can not, then you will need to install and AFS client.
Then you just need to set up GSSAPI delegation. Try adding the following to your
Host *.stanford.edu GSSAPIDelegateCredentials yes GSSAPIAuthentication yes
Some additional info here: http://snap.stanford.edu/moin/SshKerberosMac, but don't install the KCT app mentioned on the page.