Did you pay online via the activation wizard? If so, go to http://billing.microsoft.com and sign in with the LiveID you used when you paid. Do you see an active account? Is there an activate/subscribe link within OneCare? What happens when you click this? What happens when you click "check for updates" within OneCare?
Activation is two parts - the signin and payment or providing of the retail PIN *and* the "entitlement" message from the server back to OneCare on your PC to complete activation.
You can contact support -
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
-steve