Unity 4.6 adding a Canvas Text onto a gameobject prefab??

A bit confusing to explain but im using a script that auto updates text on my canvas. The scripts is attached to a gameobject prefab. I cant drag the text onto the gameprefab when its not on the scene but when i add the prefab onto the scene i am able to drag the text onto the prefab inside the scene. Any ideas what how to get around this?

 using UnityEngine;
 using System.Collections;
 **using UnityEngine.UI;**
 
 public class EnemyHealthScriptCOPY : MonoBehaviour 
 {
     /// <summary>
     /// Total hitpoints
     /// </summary>
 
     public int Sp = 0;
     private int Hp = 1;

     public bool isEnemy = true;
   
     **public Text ScoreText;**


     void OnTriggerEnter2D(Collider2D collider)
     {
         // Is this a shot?
         ShotScript shot = collider.gameObject.GetComponent<ShotScript>();
         if (shot != null)
         {
             // Avoid friendly fire
             if (shot.isEnemyShot != isEnemy)
             {
                 Hp -= shot.damage;
 
                 //                // Destroy the shot
                 //                // Remember to always target the game object,
                 //                // otherwise you will just remove the script.
                 Destroy(shot.gameObject);
                 Sp += shot.damage;
                 //scoreText.text = ("Score: " + Sp);//display in my text3d "Score: plus whatever number mySore is on
                 **ScoreText.text = ("Score: " + Sp);**
 
                 if (Hp <= 0)
                 {
                     SpecialEffectsHelper.Instance.Explosion(transform.position);
                     SoundEffectsHelper.Instance.MakeExplosionSound();
                     Destroy(gameObject);
         
                 }
 
             }
         }
     }
 }