It’s not just the ssh tunnel that’s needed, though - you actually need a client that supports authenticating via SSH / rsh, or will let you do this using a third party app. MacCVS
What client(s) have you tried (with links if possible)?
Maybe this howto will work?
You need the “tunnerling CVS RSH” section
I found a bunch of interesting stuff (including that link) when I did a google for “mac cvs client ssh”
In general, I suspect that it will be much easier to find a good GUI CVS client for OS X.