Attempting to activate uptime fails

Hi there,

I'm trying to activate the uptime option in the WPMU dashboard for http://www.nsb.com. When I do so I get the message: "Your domain does not appear to be publicly accessible on the Internet. We can only monitor the uptime of public websites". The domain is however live, active and fully operational.

I'm wondering what is prohibiting me from enabling it?

Thank you kindly,

David