What do you mean that you can't add the computer? What recognizes your login? Are you clicking subscribe in OneCare and logging in? What happens next? I suspect that you mean that you are logging in, but ti now wants you to purchase a subscription.
If you have an active subscription, which is what was created when you activated on the first computer, you need to use the same LiveID on the subsequent activations. If you are arriving at a screen to purchase or provide a token, you are likely not using the same LiveID as the subscription ID.
If you don't recall the LiveID you used for your subscription, you will need to contact support who can provide that information to you using your Key and/or contact information.
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
Alternatively, sign in at http://billing.microsoft.com with email addresses that you have set up as LiveIDs and one will show your subscription listed - that would the LiveID to use to activate with.
-steve
