The procedure section of the project http://www.sciencebuddies.org/science-f ... #procedure
, shows the method for measuring the stress, which is basically to measure the length of the rubber band.
Hooke's Law is the description of how much force is applied by a spring, depending on how far the spring is extended. Basically the force is linear in extension, as long as you haven't reached the limitations of the materials.
However, all that this experiment does is measure the *relative* stresses on the knee, rather than the absolute level of force. In order to convert the length of the rubber band into something like Newtons of force, you'd have to independently measure the spring constant of the rubber band (or other spring) using some known weights.
Hope that helps. If you have a more specific question, don't hesitate to ask.