%Mission script%

Well, that'd be GUI, make a label, position it where ever, make a string that appears there, and then you can change the string through code as the player travels... You could even add a num_Missions and make it bullet pointed.

var stringHere = "Hey! This is your mission stuff";

This is probably sloppy/not working, but you get the idea.

Here You Are The Sripts

1). Attach this script to the player, Named mission

 static var theObject : int = 0;
 
 function Update ()
 {
 
 if(theObject == 3)//the notice for objective
 {
 Debug.Log("theObject is 3, Mission Sucess");//Costumesize what            
                                            //ever youlike
 }
 
 }

2). Attach this to theObject you create

 static var amout : int = 1;//this is what amout of your object
 
 function OnTriggerEnter (player : Collider)
 {
 
 if(player.gameObject.tag == "Player")//the tag of player
 {
 mission.theObject += amout;
 Destroy(this.gameObject);//Destroy the Object
 }
 
 }