Not able to click on button whose enabled proprty is false.

There are a couple ways you could do this, but why on earth are you having it click a disabled window? :)

Anyhow....you could try something like this...

function main() {
  var object = //your button here
  clickobject(object);
}

function clickobject(object){
  var x = object.screenleft + (object.width / 2);
  var y = object.screentop + (object.height / 2);
  setparentfocus(object);
  LLPlayer.MouseDown(1, x, y, 0);
  LLPlayer.MouseUp(1, x, y, 0);
}

function setparentfocus(object) {
  while (object["parent"]) {
        if (propExists(object.parent, 'SetFocus')) {
            object.parent.SetFocus();
        }
        object = object.parent;
  }
}