Unity 5.3- SceneManager

I've just upgraded to Unity 5.3, and my scripts aren't working, because Application.LoadLevel or Application.loadedLevel aren't a thing anymore (I think). How do I use SceneManager?
I need 2 different SceneManager lines of code (C#):
-One that's the same as Application.LoadLevel(Application.loadedLevel);
-One that's the same as Application.LoadLevel("MainMenu");
TY in advance.

 

Uh,where do I put them?
Here is my Death Menu Script:

Code (CSharp):

1. using UnityEngine;
2. using System.Collections;
3.  
4. public class DeathMenu : MonoBehaviour
5. {
6.     void Start ()
7.     {
8.        
9.     }
10.     public void RestartGame()
11.     {
12.         Application.LoadLevel (Application.loadedLevel);
13.        
14.     }
15.    
16.     public void QuitToMain()
17.     {
18.         Application.LoadLevel ("MainMenu");
19.     }
20. }
21.  

Here's my Main Menu script:

Code (JavaScript):

1. #pragma strict
2.  
3. var music : AudioListener;
4.  
5. function QuitGame ()
6. {
7.     Debug.Log ("Game is exiting..");
8.     Application.Quit ();
9. }
10.  
11. function StartGame (level : String)
12. {
13.      Application.LoadLevel (level);
14. }
15.  
16. function SetGameVolume (vol : float)
17. {
18.     music.volume = vol;
19. }
20.  

And here is my PauseMenu script:

Code (CSharp):

1. using UnityEngine;
2. using System.Collections;
3.  
4. public class PauseScript : MonoBehaviour {
5.  
6.     public GameObject PauseUI;
7.     private bool paused = false;
8.  
9.     void Start ()
10.     {
11.         PauseUI.SetActive (false);
12.  
13.     }
14.  
15.     public void Restart()
16.     {
17.         Application.LoadLevel (Application.loadedLevel);
18.     }
19.  
20.     public void Quit()
21.     {
22.         Application.Quit ();
23.     }
24.  
25.     public void Resume()
26.     {
27.         paused = false;
28.     }
29.  
30.  
31.     void Update ()
32.     {
33.         if(Input.GetKeyDown("escape"))
34.             {
35.             print ("escape is pressed");
36.             paused = !paused;
37.             }
38.  
39.         if(paused)
40.         {
41.             PauseUI.SetActive(true);
42.             Time.timeScale = 0;
43.  
44.         }
45.  
46.         if(!paused)
47.         {
48.             PauseUI.SetActive(false);
49.             Time.timeScale = 1;
50.         }
51.  
52.     }
53.  
54. }
55.  

 

Thank you very much

 

Is there a more detailed explanation of using SceneManager? I feel like the documentation pages still leaves me with questions. Like what's the deal with the additive loading mode? When I add a scene can I control the visibility of it? How do I use SceneManager for loading screen situations? When you set a scene to active what does that mean?

I'm making a kiosk and would either like a better feedback mechanism while loading the other scene or I would like to have all scenes loaded at the beginning and just alternate the visibility of scenes.