How can I add a button to a list?

I added a button and appears in the inspector but not on the scene, so i cant use it. Here the code.

using System;
using UnityEngine;
using System.Collections;
using System.Collections.Generic;

public class PieMenu : MonoBehaviour
{

public List<Texture> icons ;
public List<string> commands;
public List<UnityEngine.UI.Button> buttons;

public float iconSize = 64f;
public float spacing = 12f;
public float speed = 8f;
public GUISkin skin;

[HideInInspector]
public float scale;
[HideInInspector]
public float angle;

PieMenuManager manager;

void Awake () {
	manager = PieMenuManager.Instance;
}

void OnMouseOver() {
	manager.Show(this);
}

}

It seems you have declared your list but not initialised it, or added anything to it.