VNC access to Snow Leopard OSX Server

In my department I am running SL server.
I can access it with screen sharing from my macs, and I can ssh to it
from iSSH
on my iPhone.
I cannot, however, tunnel to screen sharing through ssh.
I get an error:
Unable to initialize connection to VNC server: localhost on port 5900,
error 'connection toVNC server failed with message'.
What and where do I need to change the configuration.

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