Most people with advanced kidney disease can and should develop an activity plan tailored to their personal needs. The NKF has a very good response to this question: http://www.kidney.org/atoz/content/stayfit.cfm
In summary, exercise will keep your muscles in tone and stimulate your bones. With kidney disease, stimulating the bones causes them to retain and accrue calcium. Thus, there is less potential for excess calcium to aggregate in blood vessels. Bones lose mass when one goes for prolonged periods without activity. This was noticed during space travel, when astranauts who were exceptionally fit, returned from a zero gravity environment with decreased bone mass. It makes sense. Even notice how thin the bones are in fish, who swim in a virtually gravity free environment, compared with those of land animals. The effect of "diminished mechanical forces" has been demonstrated to decrease bone cell viability and result in bone resporption. (J Bone Miner Res. 2006 Apr;21(4):605-15) Persons who do not remain active have a much higher risk of hip and vertebral fractures.
In addition to keeping your bones and muscles in better condition and possibly reducing calcium deposition in the arteries, exercise may reduce inflammation. This was suggested in a large, government sponsored, survey - NHANES III. It showed that the odds for markers of inflammation, such as the C-reactive protein, decreased with levels of exercise in the general population (Epidemiology. 2002 Sep;13(5):561-8.). Since inflammatory markers such as the C-reactive protein are associated with a higher mortality in CKD patients(Nephrol Dial Transplant. 2008 Nov;23(11):3546-53), it seems reasonable to include exercise as part of a regimen to reduce this risk.
Check with your doctor regarding details. Each case is different. If you are a beginner, take it easy at first. Do not "over do it" to avoid an strain or injury that could set you back. One should never try to lift weights when not properly warmed up, and should increase a weight lifting regimen slowly and steadily over a regimented time course. Likewise, one should not try to bike or run at too fast a pace when starting. Do not exercise immediately after a heavy meal. Do not exercise if you have a fever or the flu. Stop exercising if you are tired, have pain, feel sick, have joint aching or muscle stiffness, chest pain or shortness of breath, or feel dizzy. When beginning your exercise, it is always best to warm up.