instead of

torus.particleEmitter.enabled = false;

you should do

torus.particleEmitter.emit = false;

and to turn it on, you would do

torus.particleEmitter.emit = true;

so your whole script according to what's currently posted, would look like:

var torus : GameObject; 

  function Start () 
  {
     torus.particleEmitter.emit = false;
  }

  function Update ()
  {
     if(Input.GetButton("Jump"))
    {torus.particleEmitter.emit = true;}
  else
  {
     torus.particleEmitter.emit = false;}
  }