Install the trial from
http://onecare.live.com and the click subscribe after installation. If you already have a subscription, enter the LiveID for your subscription and the trial will activate to your subscription. If you do not have a susbcription yet and have a new retail disc that you are trying to use, sign in with a LiveID, select the option to subscribe using a key/token and enter the key from the label behind the CD tray.
-steve
Microsoft MVP Windows Live / Windows Live OneCare, Live Mesh, & MS Security Essentials Forums Moderator