If you have a retail copy of OneCare and are trying to activate with the product key/PIN, it is located on a yellow/orange sticker inside the box, on or behind the CD holder, depending on the version of the packaging you have.
If you purchased online, there is no PIN.
If you are looking for the Product ID for trying to validate your subscription for a support request, the PID is located in Help/About inside OneCare.
-steve