The Fit-Ball straight arm pull down is predicated on the ability to stand on a *Fit-Ball, master that and you are golden. Place your Fit-Ball within the vicinity of a high cable bar. Take a four point stance on the ball, 2 shins, two hands. Shift your weight far enough forward so you can bring on foot flat on the ball, repeat with the other foot. That will leave you in a kind of frog squat position, from there careful stand until you are upright (and balanced) atop the Fit-Ball. 

Reach out and grab the high bar. Stand tall tuck your pelvis slightly forward until you feel your low abs engage, from there exhale and with amazingly straight arms draw the bar to your thighs. Exhale as you return the bar to shoulder height slowly under complete control.

Repeat as often as necessary, I usually like 3 or 4 sets of 20 where at #18 you are thinking "shoot! that's a little too heavy".

*This is an excellent exercise for those too wise to try standing on a Fit-ball, the exercise technique is exactly the same when standing on the floor in full communion with mother earth.