The plank work is tough because the upper body is already fried from the weights. Cathe has suggested tackling the plank work separately from the upper body work if it is troublesome, so if you have the DVD and can cue it up fast, practice on days when your upper body can handle the planks. Also, really concentrate on pulling your navel up and into your spine to engage the abdominals every time you roll the ball in to keep the core from collapsing.
Keep trying - you'll get there!
-Roe