You need to remove the ball adapter from the bottom of the head. Tighten the panning function of the head with the lever by the bubble level on the head. Then simply unscrew the ball from the head by gripping it with your hand and rotating the ball counter clockwise (ball side facing up). You'll need to have a good grip on it. You also may need to loosen the three flat head screws that you'll see when holding the head bottom up, those are sometimes tightened down to avoid the head from unscrewing while operating.
Then the pocket dolly should have came with a disc that has male 3/8"-16 on both side, attach that to the sled, and then the 501 to that.