The only thing I can say is experiment with different exercises. Determine which exercise DOES NOT give you problems. If you perform one of these exercises and it hurts. . . try another one until you find something that is comfortable. not quite sound advice. pain is a good indication but seeking out definitive advice regarding that condition is more in order.
Squats (w/barbell or dumbbell)
Leg Presses
Vertical Leg Presses
Leg Extensions
Hack Squats
Lunges
Sissy Squats
Jefferson Lift (a very old exercise look it up)
leg extensions and sissy squats are definitely out, due to the extreme pressure and shearing forces the patella undegoes in these movments. lunges would be a stretch too, if the knee hinges forward on the lunge. but if done properly, it can be alright.
working through a painfree range of motion on a natural squatting movement (one that distributes the weight between the upper thigh and the glutes would be the best way to go so you have knee stability (from behind) and also hip drive on your jump not just knee extension. its a chain of movement which isnt isolated just to quads to keep the motions you do as similar to that motion as possible. since your jump will be ballistic in nature, you want to start off lifting explosively (careful at first) and eventually implement some light plyometrics.
if you have "severe" tendonitis you should see a sports specific doc anyways and go off his recommendation.
I am sure that one of these exercises will not aggravate the patella. Good luck!