If you installed the beta you should have seen this on the download page:
Note for Beta participants: Unless you have received an e-mail containing instructions for activation, please do not activate Windows Live OneCare at this time. You will receive instructions for activating your free trial at the end of the Beta period.
If you have a subscription and would like to reinstall One Care and activate, download One Care 1.6 from here - http://onecare.live.com/standard/en-us/default.htm