Well, an angle bisector is a ray that "bisects" or cuts an angle in half, so, 64/2 = 32. Based off of my understanding of this picture, I would say that you have to drag the dot from point D to about halfway between each side, or, if it gives you the measurement while your moving it, move it to 32° on each side. If that doesn't work, I would try watching the help video :)
