instantiate uGUI prefab

CanvasManager.cs

using UnityEngine;

public class CanvasManager : MonoBehaviour
{
    GameObject buttonPrefab;

    void Awake ()
    {
        buttonPrefab = Resources.Load<GameObject> ("ugui/Button");
    }

    void Start ()
    {
        var instance = Instantiate (buttonPrefab);
        instance.transform.SetParent (gameObject.transform, false);
    }
}