can't use EventSystems, Reference is missing

Hi, I have a problem. I have Unity 5.1.1 and when I am trying to have "using UnityEngine.EventSystems" the reference is missing. I am trying to use it to get the object being dropped on ondrop function. I couldn't find a lot of info about this issue. Does anyone know how to fix that? Here is the code:

 using UnityEngine;
 using System.Collections;
 using UnityEngine.UI;
 using UnityEngine.EventSystems;
 
 public class Slot : MonoBehaviour {
     public int slotIndex;
     public Image icon;
 
     // Use this for initialization
     void Start () {
         icon=this.gameObject.GetComponent<Image>();
     }
 
     // Update is called once per frame
     void Update () {
         if(Control.programSlots[slotIndex]==ProgramRunner.forLoop)
         {
             icon.color=Color.blue;
         }
         
         if(Control.programSlots[slotIndex]==ProgramRunner.moveBack)
         {
             icon.color=Color.red;
         }
         
         if(Control.programSlots[slotIndex]==ProgramRunner.moveForward)
         {
             icon.color=Color.yellow;
         }
         
         if(Control.programSlots[slotIndex]==ProgramRunner.moveLeft)
         {
             icon.color=Color.black;
         }
         
         if(Control.programSlots[slotIndex]==ProgramRunner.moveRight)
         {
             icon.color=Color.green;
         }
         
         if(Control.programSlots[slotIndex]==ProgramRunner.blankAction)
         {
             icon.color=Color.white;
         }
     }
 
     public void OnDrop(EventSystems.PointerEventData data)
     {
         Control.programSlots[slotIndex]=Control.selectedCard.action;
         Control.requestSlots[slotIndex]=Control.selectedCard.req;
     }
 }

And here is the error: Assets/Slot.cs(41,28): error CS0246: The type or namespace name `EventSystems' could not be found. Are you missing a using directive or an assembly reference?

Help Please!