Some of the moves Cathe does with the ball are definitely advanced, and unfortunately, she doesn't show a less advanced ball option. It's easier to figure out alternatives if you have some stability ball experience. There are some other stability ball workouts on the market that could show you some alternate moves, or you might be able to find some info on the Internet about the moves. Or just ask here about specific moves and we can help!
One quick way to tweak the moves is to deflate your ball a bit. The firmer the ball is, the harder the workout is. By taking out a bit of air, you create a more stable base. Then you can add air bit-by-bit as you improve.
For some moves where you are face down and in a plank position, you can create a more stable base by having the ball farther up your leg, starting with being positioned so your quads are on the ball, the going to just above the knee, then just below the knee (avoid having the knee right over the ball to protect the joint), then on your shins, then on your ankles.
For pikes (which I haven't been able to master yet, seemingly because of my wrists and shoulder more than core strength), you can stay with a "knee-in" move while they go to shoulder planks. As you progress, you can hoist your butt up a bit higher.
It IS hard to get your balance on the stability ball (or should we say "INstability ball" LOL) when you first start out. But it is definitely worth it to try the stability ball and to work up to the harder moves. Try just sitting on the ball while you watch TV or when you are at your desk. This can help strengthen the core on its own and get you used to being comfortable on the ball.
I am currently doing a "Core Foundations" workout by another instructor, and the first time I did it, I couldn't do some moves, and had to modify several of the others. I definitely was frustrated, and felt like a wimp, even though, aside from those shoulder pikes, I could do Cathe's ball work. I kept at it, though, and have since done the workout 4 times (once or twice a week), getting better each time.
Hang in there!
...Carole