Doubt on using in Visual Studio

I create a new project but I can not put those using as in this example below. It appears as in the other code below ... Please help me = /

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Runtime.InteropServices.WindowsRuntime;
using Windows.Foundation;
using Windows.Foundation.Collections;
using Windows.UI.Xaml;
using Windows.UI.Xaml.Controls;
using Windows.UI.Xaml.Controls.Primitives;
using Windows.UI.Xaml.Data;
using Windows.UI.Xaml.Input;
using Windows.UI.Xaml.Media;
using Windows.UI.Xaml.Navigation;
using Windows.UI.Popups; // Não esqueça de adicionar este using

// The Blank Page item template is documented at http://go.microsoft.com/fwlink/?        LinkId=234238

namespace App1
{
/// <summary>
/// An empty page that can be used on its own or navigated to within a Frame.
/// </summary>
public sealed partial class MainPage : Page
{
    public MainPage()
    {
        this.InitializeComponent();
    }

    private async void Button_Click(object sender, Windows.UI.Xaml.RoutedEventArgs e)
    {
        // Aqui crio uma mensagem de dialog e a exibo.
        MessageDialog dialog = new MessageDialog("Teste", "Teste Dialog");
        await dialog.ShowAsync();
    }
}
}

It starts like this one:

using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

namespace WpfApplication3
{
/// <summary>
/// Interaction logic for MainWindow.xaml
/// </summary>
public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
    }

    private void Button_Click(object sender, RoutedEventArgs e)
    {
        MessageBox.Show("Teste");
    }
}
}