If you're new to using a stability ball, it's absolutely normal to be unstable (they should call it an "instability ball"LOL!).
The good news is, using the ball will really stengthen your own stablizing muscles, and it gets easier every time you try. You can stabilizes the ball a bit at first by setting it in an upside-down step riser ( I just thought of that, pretty neat, huh?).
Roll-ins and jack knives are some of the most difficult, if not THE most difficult stability ball moves, so don't be discouraged that you can't do them. I wasn't able to do jackknives until after I did a rotation of P90X and strengthened my core and sholder girdle. There's a lot that needs to work to make this move work: core, glutes, arms, shoulder girdle.
Just keep on trying, and be happy with small improvement.