I'm not clear on what you are being directed to. To activate on a second PC, you simply open OneCare, click on subscribe/activate and enter the LiveID for your subscription (open OneCare, click Help/About on the already activated first PC to see the Subscription ID). If you encounter problems with this, please contact OneCare support.
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
-steve