The livestrong.com website makes it probably as easy as it can be. You probably will have to really pay attention to the measurements but it makes it so that you can enter things in 1/4 or 1/2 or whatever of the amount it says is a 'serving'. So you could enter .25 servings or 5.75 servings or whatever. It might not be exact but it should get pretty close.
Sorry to hear that being active is painful. Obviously I am not familiar with your particular issue but maybe you just haven't found "your thing" yet?