How to move an object around with the arrow keys (Javascript)

Here's a very simple script to show you the basics. Put it on each object you want to move. Make sure that each object has a collider so it can catch the OnMouseDown().

 static var selectedId : int;
 static var speed : int = 5;
 
 function Update () {
     if (selectedId==GetInstanceID()) {
         if (Input.GetKey (KeyCode.UpArrow)) transform.Translate (Vector3(0,1,0) * Time.deltaTime*speed);
         if (Input.GetKey (KeyCode.DownArrow)) transform.Translate (Vector3(0,-1,0) * Time.deltaTime*speed);
         if (Input.GetKey (KeyCode.LeftArrow)) transform.Translate (Vector3(-1,0,0) * Time.deltaTime*speed);
         if (Input.GetKey (KeyCode.RightArrow)) transform.Translate (Vector3(1,0,0) * Time.deltaTime*speed);
     }
 }
 
 function OnMouseDown () {
     selectedId = GetInstanceID();
 }

selectedId is made static so it will remain as a single variable, that way only one item can match the id. OnMouseDown() is equal to sending a raycast towards the object, therefore it must have a collider otherwise the ray will pass through. Also have a look at the input class to learn about inputs and steering.