This Unity script will add fields in the inspector that allow you to enter a scene number. When the user presses the space bar, Unity will load the new scene.

SceneLoader.cs

using UnityEngine;
using System.Collections;
using UnityEngine.UI;

public class SceneLoader : MonoBehaviour {

    private bool loadScene = false;

    [SerializeField]
    private int scene;
    [SerializeField]
    private Text loadingText;


    // Updates once per frame
    void Update() {

        // If the player has pressed the space bar and a new scene is not loading yet...
        if (Input.GetKeyUp(KeyCode.Space) && loadScene == false) {

            // ...set the loadScene boolean to true to prevent loading a new scene more than once...
            loadScene = true;

            // ...change the instruction text to read "Loading..."
            loadingText.text = "Loading...";

            // ...and start a coroutine that will load the desired scene.
            StartCoroutine(LoadNewScene());

        }

        // If the new scene has started loading...
        if (loadScene == true) {

            // ...then pulse the transparency of the loading text to let the player know that the computer is still working.
            loadingText.color = new Color(loadingText.color.r, loadingText.color.g, loadingText.color.b, Mathf.PingPong(Time.time, 1));

        }

    }


    // The coroutine runs on its own at the same time as Update() and takes an integer indicating which scene to load.
    IEnumerator LoadNewScene() {

        // This line waits for 3 seconds before executing the next line in the coroutine.
        // This line is only necessary for this demo. The scenes are so simple that they load too fast to read the "Loading..." text.
        yield return new WaitForSeconds(3);

        // Start an asynchronous operation to load the scene that was passed to the LoadNewScene coroutine.
        AsyncOperation async = Application.LoadLevelAsync(scene);

        // While the asynchronous operation to load the new scene is not yet complete, continue waiting until it's done.
        while (!async.isDone) {
            yield return null;
        }

    }

}