i would look more at correcting the cause of the imbalance rather than just what can be done to compensate for it. also, are you positive that you have an imbalance or just guessing that that is why your knee cap is off center? when you walk, are your toes pointing straight ahead or are they pointing in or outwards?
if you truly do have an imbalance, the above mentioned exercises are all quad strengtheners, but you could also try an adduction exercise
