Public Shared Sub AddMouseDownHandler ( _
    element As DependencyObject, _
    handler As MouseButtonEventHandler _
)

Dim element As DependencyObject
Dim handler As MouseButtonEventHandler

Mouse.AddMouseDownHandler(element, _
    handler)

public static void AddMouseDownHandler(
    DependencyObject element,
    MouseButtonEventHandler handler
)

public:
static void AddMouseDownHandler(
    DependencyObject^ element, 
    MouseButtonEventHandler^ handler
)

public static function AddMouseDownHandler(
    element : DependencyObject, 
    handler : MouseButtonEventHandler
)

You cannot use methods in XAML.