On the PC that you wish to add OneCare to, install OneCare from http://onecare.live.com and then click subscribe within the newly installed OneCare. Subscribe using the same LiveID used to activate the subscription on the first PC.
Clicking Add PC in the Manage OneCare Circle dialog on a hub PC does not add a PC to a subscription, it should bring up text telling you to go to the other PC to install and activate.
-steve