AdManager is a script copied from Unity Technologies which allows you to add Ads integration very easy

AdManager.cs

using UnityEngine;
using System.Collections;
using UnityEngine.Advertisements;

public class AdManager : MonoBehaviour 
{
    //Important: Change the gameID to your game's
    //id in the unityads.unity3d.com dashboard
    [SerializeField] string gameID = "33675";

    void Awake()
    {
        Advertisement.Initialize (gameID, true);
    }

    public void ShowAd(string zone = "")
    {
        //We simulate the mobile's behaviour.
        //It results to be that in the editor, showing
        //the ad doesn't stop the game for playing
        //in the background.
        #if UNITY_EDITOR
            StartCoroutine(WaitForAd ());
        #endif

        if (string.Equals (zone, ""))
            zone = null;

        ShowOptions options = new ShowOptions ();
        options.resultCallback = AdCallbackhandler;

        if (Advertisement.isReady (zone))
            Advertisement.Show (zone, options);
    }

    void AdCallbackhandler (ShowResult result)
    {
        switch(result)
        {
        case ShowResult.Finished:
            Debug.Log ("Ad Finished. Rewarding player...");
            break;
        case ShowResult.Skipped:
            Debug.Log ("Ad skipped. Son, I am dissapointed in you");
            break;
        case ShowResult.Failed:
            Debug.Log("I swear this has never happened to me before");
            break;
        }
    }

    IEnumerator WaitForAd()
    {
        float currentTimeScale = Time.timeScale;
        Time.timeScale = 0f;
        yield return null;

        while (Advertisement.isShowing)
            yield return null;

        Time.timeScale = currentTimeScale;
    }
}