Product review (4 stars): Fortunately sears has a picture of the mullion on their ...
I purchased this spring to for my Kenmore Elite refrigerator (model #79571073011).  The spring fits in the bottom bracket of the "Articulating mullion".  It was a few dollars more on Amazon then on Sears parts direct, but free shipping with Amazon prime made it cheaper.

It wasn't obvious where the spring should be installed.  Fortunately sears has a picture of the mullion on their website and I could see where the spring is installed.  As another poster mentioned, you need to remove a piece of plastic that holds the bottom 'flipper' (hinge) in place that the springs goes into (and put it back in place after getting the spring installed).

I was able to unscrew the middle section, hold the mullion and install the spring at the bottom hinge without unplugging the wires between the mullion and the door.  The spring seems to sit awkwardly when the mullion is in the open position, but you don't see it when the mullion is closed.  If you can do that, then just slide the mullion back on the door, screw the middle hinge back in place and you are good to go!