First, seek professional help on rehabilitating the knee. We need more specific information to help with that aspect. When exactly did the injury occur? Precisely what are you doing to rehabilitate now? Specifically which exercises cause no pain and specifically which exercises cause pain?
For CARDIO, non-impact exercises are what you are looking for. Swimming and rowing are best as those are the 2 functional, non-impact, cardio exercises you can do. I would suggest biking, but that depends on your range of motion in the knee.