Particle effect instantiate problem.

 //Enemy Script
 
 //Inspector variables
 var numberOfClicks : int =2; //no of times you click to destroy object
 var respawnWaitTime : float =2.0;
 var shapeColour :Color[]; //colour array
 var explosion :Transform; //loads the particle effect
 var enemyPoints : int =1; // value of enemy destroyed
 
 //Private variables
 private var storeClicks : int =0;
 
 function start()
 {
     storeClicks = numberOfClicks;
 }
 
 function Update ()
 {
     if(numberOfClicks <=0)
     {
         if(explosion)
         {
             Instantiate (explosion, transform.position, transform.rotation); //create an explosion
         }
         var position = Vector3 (Random.Range(-6,6),Random.Range(-4,4),0); //new random position for game objects
         RespawnWaitTime ();
         transform.position = position; //move game object to new position
         numberOfClicks =storeClicks;
         }
     }
 // will be used to hide and then unhide a GO for a random amount of time
 function RespawnWaitTime ()
 {
     renderer.enabled =false;
     RandomColour();
     yield WaitForSeconds (respawnWaitTime);
     renderer.enabled =true;
 }
 // random colour is used to change the colour of enemy GO in game
 function RandomColour ()
 {
     if(shapeColour.length >0)
     {
     var newColour = Random.Range(0,shapeColour.length);
     renderer.material.color =shapeColour[newColour];
     }
 }

Been following the Walker Boys tutorial for a basic clicking game, everything seemed to be going smoothly until I got to adding lines of code for creating a scoring system. Could this be an issue with using unity 4 and the tutorials being 2 years old? Or is it just me and my lack of understanding?

Any help appreciated as always.