Open OneCare on the PC you wish to make a hub PC.
Click on Manage Your OneCare Circle.
Click on "Make this PC a hub."
Sign in with your subscription LiveID and password (the LiveID should already be filled in)
A dialog appears that reports "This computer is now a Hub PC."
Note that the above will only work if you have more than one computer activated with OneCare 2.0 using the *same* subscription LiveID.
-steve