Please know that a TENS machine is not for muscle stimulation. It is used for pain management. If your quad muscles are not functioning properly at this point in your recovery, it is not recommended that you try and "jolt" them into action with stimulation. It is not appropriate to use the EMS machine in this manner either. You risk doing damage when you try to make your body do something it is not ready to do yet.
Some quads just take time. Be patient! Yours will activate when they are ready. Do the exercises with both legs and when the time is right, both legs will work.