Universal Apps MessageBox: "The name 'MessageBox' does not exist in the current context"

For Universal Apps, the new APIs require you to use await MessageDialog().ShowAsync() (in Windows.UI.Popups) to bring it into line with Win 8.1.

var dialog = new MessageDialog("Your message here");
await dialog.ShowAsync();

Just wanted to add to ZombieSheep's answer: also, customization is quite simple

        var dialog = new MessageDialog("Are you sure?");
        dialog.Title = "Really?";
        dialog.Commands.Add(new UICommand { Label = "Ok", Id = 0 });
        dialog.Commands.Add(new UICommand { Label = "Cancel", Id = 1 });
        var res = await dialog.ShowAsync();

        if ((int)res.Id == 0)
        { *** }

try this:

 using Windows.UI.Popups;

code:

private async void Button_Click(object sender, RoutedEventArgs e)
    {

        MessageDialog msgbox = new MessageDialog("Would you like to greet the world with a \"Hello, world\"?", "My App");

        msgbox.Commands.Clear();
        msgbox.Commands.Add(new UICommand { Label = "Yes", Id = 0 });
        msgbox.Commands.Add(new UICommand { Label = "No", Id = 1});
        msgbox.Commands.Add(new UICommand { Label = "Cancel", Id = 2 });

        var res = await msgbox.ShowAsync(); 

        if ((int)res.Id == 0)
        {
            MessageDialog msgbox2 = new MessageDialog("Hello to you too! :)", "User Response");
            await msgbox2.ShowAsync();
        }

        if ((int)res.Id == 1)
        {
            MessageDialog msgbox2 = new MessageDialog("Oh well, too bad! :(", "User Response");
            await msgbox2.ShowAsync();
        }

        if ((int)res.Id == 2)
        {
            MessageDialog msgbox2 = new MessageDialog("Nevermind then... :|", "User Response");
            await msgbox2.ShowAsync();
        }


    }

To Trigger some Function When "Yes" or "No" is clicked, You can also use:

msgbox.Commands.Add(new UICommand("Yes", new UICommandInvokedHandler(this.TriggerThisFunctionForYes)));
msgbox.Commands.Add(new UICommand("No", new UICommandInvokedHandler(this.TriggerThisFunctionForNo)));