Taking high score from anther script and then displaying it

Hi. In my game the player is climbing up a wall while objects are trying to knock him off. His score is how many seconds he has survived for. I am trying to make a high score but its not going so well. Here is what I have now:

 using UnityEngine;
 using System.Collections;
 
 public class AddToScore : MonoBehaviour {
 
     private int now = 3; 
     public int score = 0; 
     private string second = " second";
     private string seconds = " seconds"; 
     public int highScore = 0; 
     public GUIText scoreLabel; 
 
 
     void Start(){
         scoreLabel.text = score.ToString() + seconds;
         InvokeRepeating ("AdToScore", 1, 1);
     }
 
     void Update(){
         if (score > 0) {
             second = " seconds"; 
         }
 
         if (score > highScore) {
             PlayerPrefs.SetInt("highscore", score);
             PlayerPrefs.Save();
             highScore = score; 
         }
     }
 
     void AdToScore(){
         if (now > 0) {
             score = score + 1; 
             scoreLabel.text = score.ToString() + second;
 
         }
     }
 }
 

I am sure there is something wrong with creating and saving this high score because it is not displaying in a text mesh. Here is the code for that:

 using UnityEngine;
 using System.Collections;
 
 public class DisplayHighScore : MonoBehaviour {
 
     public TextMesh highScore78;
     private AddToScore addtoscoreReference; 
 
     // Use this for initialization
     void Start () {
     
         PlayerPrefs.GetInt ("highScore");
 
         highScore78.text = addtoscoreReference.highScore + " seconds"; 
 
     }
     
     // Update is called once per frame
     void Update () {
     
     }
 }

Please ignore the fact that there is a 78 in the variable name for the text mesh. I just use it to differentiate from other variables. Anyways please help me. Thanks!