Just by adding motion tweens between frames 1, 9 and 17, your arms should be animated swinging back and forth like below.



But as I mentioned in the overview, the body moves up and down but the arms don't. This requires adding keyframes and nudging the arm symbol either up or down, depending on where the body is on that frame. You will need to do this for all the odd frames (but leave frames 1, 9 and 17 alone - they should not need adjustment).

1. If you followed directions correctly, the body has a keyframe on every odd frame, 1,3,5,7, etc. For both arm layers, add a keyframe on frames 3, 5 and 7.
2. Go to keyframe 3 and select the arm symbol. Using the down arrow key on your keyboard, nudge the arm down until it is in the correct position.
3. Do the same for frames 5, 7, 11, 13, and 15, and nudge the arms up or down appropriately. Now your arms should swing and move up and down like so.
   
4. Now's the best time to test your Walk Cycle