leg press machines will often aggravate knees, not help them. My recommendation is to do single-leg squats. You can do them with bodyweight and they will definately help build muscle mass. Also step ups will help the tissue around the knee. From there, proceed to weighted squats and such -but stay of the leg press!

I powerlift in the off-season, and my legs don't get much bigger, but they do get stronger, which is always the more important thing. My legs are tree trunks now anyway.