How to pass a value in main page to second page in windows phone app 8.1

1) C# is a case-sensitive language, so if you would write

using system.windows.navigation

It won't work, you have to use System.Windows.Navigation[^] namespace for your project. Just make sure you are referencing the DLL also, Double-click on References and see what DLLs are present.

2) I would like to have the data stored in the App context so that both of the pages can work around with the data that was passed.

public static object Data { get; set; }

Which can then be altered as,

// Before navigating
App.Data = "String data";
rootFrame.Navigate(typeof(Page2));

// After navigation
var data = (string)App.Data; // Get the data

This way you will be able to pass the data from one page to another, but also maintains the data in the Application context.

3) MessageBox doesn't exist in Metro applications. You get to preview the message as a dialog. A message dialog in Metro application is MessageDialog[^] object.

new MessageDialog("Message to show").ShowAsync();

Note that these are asynchronous models, you would have to apply an await with them.