If weight loss would allow your dog to have the cruciate surgery safely than you should get her to lose weight quickly, but safely, of course. A low to no carb and low fat diet is the fastest way for a dog to lose weight, and would not affect the Addison's disease.
Exercise and physical therapy will help her to lose weight even faster. If there is a veterinary physical therapist in your area it would be a good idea to have a consult and be taught by the physical therapist appropriate exercises that can be performed at home. The underwater treadmill is probably the best exercise apparatus that your dog should be using right now, but they are costly and probably only available at the physical therapists. You could walk your dog around in circles using the wading pool. If you are near another water body walk your dog in the water as often as possible. The water should be just high enough so that your dog is halfway submerged, with her head fully extended so that she is buoyant, and won't put any strain on her knees.
Prolotherapy is the injection of small amounts of nutritive substances into ligaments, tendons and other joint components to aid in the repair of that joint. It may not help with a complete tear of a cruciate ligament, but it can help with incomplete tears. It certainly would not hurt to give it a try if it is available in your area, especially on the right stifle.
Thank you very much.. I have found an animal physiotherapist at an animal emergency centre and have made an appointment for Tina to see her. She is on a very strict diet now, and tries to eat the oats that I throw out for my chickens. The chickens always win that battle. She really has got personality plus. She hasn't had an easy life but she is still a happy, inquisitive little dog.
Thanks again,
Chirley