1) Make sure you're squatting properly. I mean this in the least offensive way possible, but if you're using a medicine ball to squat, you're probably not doing it right -- just an observation from the unlimited supply of bad squats I've seen in my life. Whatever squat variation you use, you should be getting your hips below your knees at the bottom of the movement, while keeping your weight on your heels and your back in proper extension. If you use a medicine ball, kettlebell or dumbbell at your chest, hold the weight right up against your chest. The further away you have the weight, the more it messes things up and decreases productivity with the lift. Even better than a medicine ball (or similar style weight) would be barbell back squats, which engage the posterior chain a lot more (because you have to sit back more to keep the weight over your heels which increases movement and contraction in the glutes and hamstrings) and allow the heaviest load you can do through a full range of motion squat. You won't use full ROM when jumping - it's impractical to do so - but the full depth squat will give the most bang for your training buck when it comes to squats.
2.1) As others have said, deadlift. A conventional deadlift is the single heaviest freeweight exercise a person of average skeletal proportions can perform, and it uses your posterior chain even more than the squat. It's worth mentioning that posterior chain strength and power is one of the most important things for building a good jump, other than actually jumping of course.
2.2) Do deadlifts for both max effort (heavy sets of 1-5 reps, but only as heavy as you can do with good form) and dynamic effort (lots of fast sets at 1-5 reps at ~half the weight you'd use for max effort) to get the best effect. Other dynamic lifts that take on a similar form to deadlifts and are worth doing include jump shrugs and clean pulls. Olympic style weightlifting is very beneficial to vertical jumps, and plenty of Oly lifters can jump very high because of their training, but you don't need to learn to actually clean or snatch a bar. It's really the explosive second pull in weightlifting (which is the fast part of the lift pulling the bar from the mid-thighs and up until the arms unlock) that contributes most to vertical jump strength in this instance.
3) Obviously, practice vertical jumps.
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