Hola Jorge.
Para mostrar un MessageBox en una App para Windows Store tienes que usar esto
Private Async Sub CloseCommandLaunch_Click(sender As Object, e As RoutedEventArgs)
' Create the message dialog and set its content and title
Dim messageDialog = New MessageDialog("No internet connection has been found.")
' Add buttons and set their callbacks
messageDialog.Commands.Add(New UICommand("Try again", Sub(command)
rootPage.NotifyUser("The '" & command.Label & "' button has been selected.", _
NotifyType.StatusMessage)
End Sub))
messageDialog.Commands.Add(New UICommand("Close", Sub(command)
rootPage.NotifyUser("The '" & command.Label & "' button has been selected.", _
NotifyType.StatusMessage)
End Sub))
' Set the command that will be invoked by default
messageDialog.DefaultCommandIndex = 0
' Set the command to be invoked when escape is pressed
messageDialog.CancelCommandIndex = 1
' Show the message dialog
Await messageDialog.ShowAsync
End Sub
http://msdn.microsoft.com/en-us/library/windows/apps/windows.ui.popups.messagedialog.aspx?cs-save-lang=1&cs-lang=vb#code-snippet-2
"En los momentos de crisis, sólo la imaginación es más importante que el conocimiento"
MCTS | SharePoint 2010, Application Development
MCTS | SharePoint 2010, Configuring
MS | Programming in HTML5 with JavaScript and CSS3 (MS), Developing ASP.NET MVC 4 Web Applications