UnityEngine.WSA Not found

Hello,

I am building a Windows Store application using the below script. I am getting error Error 5 The type or namespace name 'WSA' does not exist in the namespace 'UnityEngine' (are you missing an assembly reference?) I am still learning Unity and C#, but I once had the same issue in another app but some how I got it working. How do I get the WSA added into the UnityEngine?

Thanks,

John

code:

 using UnityEngine;
 #if UNITY_WINRT
 using System;
 using System.Collections;
 using System.Threading;
 using System.IO;
 
 
 
 /// <summary>
 /// Windows specific and interop between Unity and Windows Store or Windows Phone 8
 /// </summary>
 public static class WindowsGateway
 {
     //notes
     static WindowsGateway()
     {
 #if UNITY_METRO
         // unity now supports handling size changed in 4.3
         //UnityEngine.WSA.Application.windowSizeChanged += WindowSizeChanged;
         UnityEngine.WSA.Application.windowSizeChanged += WindowSizeChanged;
 #endif
 
         // create blank implementations to avoid errors within editor
         UnityLoaded = delegate { };
 
         //blank implementation for our Share function
         ShareHighScore = delegate { };
 
     }
 
     /// <summary>
     /// Called from Unity when the app is responsive and ready for play
     /// </summary>
     public static Action UnityLoaded;
 
     //called when we want to Share a high score
     public static Action ShareHighScore;
 
 #if UNITY_METRO
     /// <summary>
     /// Deal with windows resizing
     /// </summary>
     public static void WindowSizeChanged(int width, int height)
     {
         if (width <= GameManager.Instance.startingScreenSize.Width / 2)
         {
             //GameManager.Instance.paused();
             Pause.paused = true;
 
         }
         else
         {
             //GameManager.Instance.unpaused();
             Pause.paused = false;
         }
 
     }
 #endif
 
 }
 
 #endif