How to make a selectable platform?

try this.. though some restrictions are not yet set.. but selecting the platform and moving are all working..

Attach this to your player.I named that script "Script" important for variable calling. add a rigidbody2d component. set gravity to 0. also add a circle collider2d for trigger function. check is trigger.

 #pragma strict
 static var platformSelect : boolean;  //will be accessed by other script
 
 var moveUp: KeyCode;
 
 var moveDown: KeyCode;
 
 var moveLeft: KeyCode;
 
 var moveRight: KeyCode;
 
 var speed: float = 4;
 
 function Update () { 
 
     if(!platformSelect) {  //if platformSelect is false player can move
         if (Input.GetKey(moveUp)) { 
             rigidbody2D.velocity.y = speed; }
     
         else if (Input.GetKey(moveDown)) { 
             rigidbody2D.velocity.y = speed * -1; }
 
         else if (Input.GetKey(moveLeft)) { 
             rigidbody2D.velocity.x = speed * -1; }
 
         else if (Input.GetKey(moveRight)) {
             rigidbody2D.velocity.x = speed; }
 
         else { rigidbody2D.velocity.y = 0;
             rigidbody2D.velocity.x = 0; }
     }
     
 }

now for your cube "platform" add this script and make it as a prefab. also add a rigidbody2d, gravity 0. also add 2 boxcollider2d. check is trigger on 1 of the box. and check is kinetic on the other one.

 #pragma strict
 var select : KeyCode;  // i used spacebar for this key
 var onPlatform : boolean;
 var player : GameObject;  // i assigned it on the function start
 var platformSelect : boolean;
 var moveUp: KeyCode;
 
 var moveDown: KeyCode;
 
 var moveLeft: KeyCode;
 
 var moveRight: KeyCode;
 
 var speed: float = 4;
 
 function Start () {
     player = GameObject.Find("Player");
 }
 
 function Update () {   //onPlatform is triggered by onTrigger, see below script
     if(onPlatform && Input.GetKeyDown(select)) {
         Debug.Log("Selected");
         platformSelect = true;
         Script.platformSelect = true;  //affects the other script. Important
         onPlatform = false;
         rigidbody2D.isKinematic = false;
     }else if(!onPlatform && Input.GetKeyDown(select)) {
         Debug.Log("unSelect");
         platformSelect = false;
         Script.platformSelect = false; // enable players movement again
         rigidbody2D.isKinematic = true;  //will stop platform on collision
     }
     
         if(platformSelect) { //if true, player will follow platform
             player.transform.position = transform.position;
             //will move the platform on keypress
             if (Input.GetKey(moveUp)) { 
             rigidbody2D.velocity.y = speed; }
     
         else if (Input.GetKey(moveDown)) { 
             rigidbody2D.velocity.y = speed * -1; }
 
         else if (Input.GetKey(moveLeft)) { 
             rigidbody2D.velocity.x = speed * -1; }
 
         else if (Input.GetKey(moveRight)) {
             rigidbody2D.velocity.x = speed; }
 
         } //removed the else velocity 0 for the nonstop effect.
     }
 
 
 
 function OnTriggerEnter2D (hitInfo:Collider2D){
     if(hitInfo.name == "Player") {
         Debug.Log ("hit");
         onPlatform = true;
     }
 }
 function OnTriggerExit2D (hitInfo:Collider2D){
     if(hitInfo.name == "Player") {
         Debug.Log ("exit");
         onPlatform = false;
     }
 }

dont mind the debug.log, you can delete it, i just used it as reference. Hope this solves your problem..