Need help with script and trigger, appearance on screen buttons

I don't think I get what you want, but maybe something like this:

  private var standsOnTrigger : boolean;

  function OnTriggerEnter(col : Collider) {

      if (col.gameObject.tag == "Player") {

          standsOnTrigger = true;

      }

  }

  function OnTriggerExit(col : Collider) {

      if (col.gameObject.tag == "Player") {

          standsOnTrigger = false;

      }

  }

  function OnGUI() {

      if (standsOnTrigger) {

          GUI.Button(...);

          ...

      }

  }