If OneCare was installed and activated by the store, then you have a subscription using a LiveID they created for you and provided to you (or should have). If, however, OneCare was in trial mode and not activated, then you needed to subscribe using your LiveID and entering the product key. You can only use the key once, though. So, your statement about having done this, but OneCare now showing as being in trial mode, would seem to indicate that you have reinstalled OneCare or have installed to another PC, in which case, you need to use the same LiveID for all subsequent activations.
If you did not activate with the key and you are getting a message about the key being used already, either return the copy of OneCare to the store for an exchange (or the LiveID used to activate by them) or contact support to determine what LiveID your subscription is associated with.
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
-steve
