Hi Annetoin! Is it just the leg press exercise that bothers your knees or any exercise that is weightbearing (ie: walking lunges, one legged squats). If it is just the leg press exercise, then you can do the other exercises I mentioned in the parenthesis.
If you cannot do weight bearing exercises, and have a gym membership, you can use the butt blaster or vertical glute press. If you do not have a gym membership then you can do the stability ball portion of my pyramid lower body tape or put an ankle weight on and do the "pizza press" motion for 3 sets of 15 to 20 reps.
Hope this helps.