Using SlimDX for Controller Input Error

Question

I'm using SlimDX to set up controller input and encountering an error with the following code snippet:

if (state.Buttons(a))
{
    // Navigate to the page, using the NavigationService
    this.NavigationService.Navigate(GameplayScreen);
}

The error message is:

Error CS1061 'MainPage' does not contain a definition for 'NavigationService' and no accessible extension method 'NavigationService' accepting a first argument of type 'MainPage' could be found (are you missing a using directive or an assembly reference?)

What could be causing this issue?