how to call a class on click

You can't call a class. A class is only a template for creating object. You define your variables and his behaviour (method/functions) for this group of object in a class.

You then need to instantiate this class to create an object.

In Unity, if your class inherit from MonoBehaviour (which is the default):

 public class MyClass : MonoBehaviour {
 
 }

you only have to attach it to a GameObject to instantiate it.

So once you have an instance of your object you can then call his function or his methods.

In your case you want to be able to display a window when you have clicked on a button. Attach both of your script on the same GameObject and modify a little bit dialog.cs to look like this:

 public class dialog : MonoBehaviour {
     public GUIStyle mystyle;
     public Rect windowRect = new Rect (150, 80, 200, 100);
  
     // Declare a boolean letting this object know when to display the window
     private bool displayDialog;
 
     public  void OnGUI () {
        // Only display the window when displayDialog is true
        if (displayDialog) {
            windowRect = GUI.Window (0, windowRect,    WindowFunction, "Save !!!");
        }
     }
  
     public  void WindowFunction (int windowID) {
          GUI.Label( new Rect( 40, 40, 120, 50 ), "Do you want to save ?? ",mystyle  );
        if (GUI.Button (new Rect (10, 70, 70, 20), "Yes ")) 
        { 
          Application.LoadLevel("Settings");
        }
  
        if (GUI.Button (new Rect (120, 70, 70, 20), "No ")) 
        { 
          Application.LoadLevel("Settings");   
        }
     }
 
     public void OpenDialog() {
         displayDialog = true;
     }
 
     public void CloseDialog() {
         displayDialog = false;
     }
  
 }

and then just add this in ur toast.cs:

 // We declare a global variable so we can access it from anywhere in this class
 public dialog dialogScript;
 
 void Start() {
     // Associate the dialog instance of this GameObject to the global variable
     dialogScript = gameObject.GetComponent<dialog>();
 }
 
 void OnGUI() {
 
     if (GUI.Button(new Rect (10, 350, 70, 20), "Back "))
     {
         // When the Back button is clicked, call the OpenWindow function on the dialog instance
         dialogScript.OpenWindow();
     }
 }