There is no charge if you're on a share everything plan. I was under the impression that the only plans charged are the grandfathered unlimited data plans.
As for your inability to get it to work, I'm not sure if you're saying that the setting isn't there, or that you're getting routed to a web page that gives you the option of adding sharing. If the latter is the case, it's a simple fix - call them on the phone. When Tango came to the Trophy, I had the same problem. A phone call, and then waiting an hour or two for the settings to propagate through their system, and I was up and running. No charge.