you must make sure the ipo curve is in object mode.  then make sure the ipo curve for the force is enabled for viewing.  to do this extend the window of the ipo curve to take up most of the window.  all the way at the right of the ipo window are some names such as Xloc Yloc and so on.  all the way at the bottom there is a object something like Fcurve or something like that (i am not infront of blender so i cant tell you.