Go to http://billing.microsoft.com and sign in with the LiveID for your subscription. Click renew for the OneCare subscription and select the option to renew with a PIN or Token. This is the product key from inside the retail package.
It could take up to 24 hours for your subscription status to be reflected in OneCare.
Note that there was no need to reinstall OneCare. When you activated the reinstall with the same LiveId as you existing subscription, it inherited the old subscription status.
-steve