Its a very simple task. select the joint and then next to the play button for sketchyphysics there is a button that says UI on it. click it and depandant on whether you have sketchyphysics 2 or 3 their will be a text field called controller. if you have sketchyphysics 2 just type what you want the slider to say. if you set 2 or more joints to the same name one slider will control both joints. in sketchyphysics 3, due to the new coding features you will have to put: slider('[slidername]')
For example if i wanted the slider to be called steer i would put:
slider('steer')
Once again if 2 or more joints have the same slider name they will both use only 1 slider.