If onecare comes bundled with your internet subscription, that means that your service provider must provide with a Live ID which you should use to activate your subscription for onecare.
Have you activated onecare with the Live ID your provider have assigned to you?
In your onecare main screen you should not have a subscribe or activate button available.
If you do:
Click on subscribe/activate
Sign in with the Live ID your ISP have provided you.
If you don't have the button:
Verify if the date and time of your computer is correct.