You should be able to click on Subscribe and sign in with the LiveID for the subscription of the Circle you are rejoining. Open OneCare on one of the other PCs in that Circle and click Help/About to see the LiveID.
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 the activation dialog already contains a LiveID from a prior attempt or one used in trial mode, but this is not the one you need to activate with to associate it with your subscription, see this post for how to reset this - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2776450&SiteID=2
-steve