WPF Treelistview with mousedown event

Hello Everyone,

starting my training, but sadly stuck on something probably everyone knows, so i am trying to make an event to happen on a child treeitemview, if user left clicks on it, I want a message box to appear, simple right?

sadly not for me :(

here is what i have so far, let me know what part i goofed up:

<TreeView x:Name="treeView" HorizontalAlignment="Left" Height="313" Margin="10,45,0,0" VerticalAlignment="Top" Width="215" Grid.ColumnSpan="2"><TreeViewItem Header="Sales" FontSize="15"><TreeViewItem Header="Customer" Name="SubTreeCustomer" MouseDown="SubTreeCustomer_MouseDown"></TreeViewItem><TreeViewItem Header="Sales Orders" Name="SubTreeSO"></TreeViewItem><TreeViewItem Header="Shipping" Name="SubTreeShip"></TreeViewItem></TreeViewItem><TreeViewItem Header="HR" FontSize="15"><TreeViewItem Header="Employee"></TreeViewItem></TreeViewItem><TreeViewItem Header="Purchasing" FontSize="15"><TreeViewItem Header="Purchase Order"/><TreeViewItem Header="Shipping" Height="21"/><TreeViewItem Header="Receiving"/></TreeViewItem>

and C#:

  public Details()
        {
            InitializeComponent();
            SubTreeCustomer.MouseDown += new MouseButtonEventHandler(SubTreeCustomer_MouseDown);
        }

        private void btn_Close_Click(object sender, RoutedEventArgs e)
        {
            this.Close();
        }

        private void SubTreeCustomer_MouseDown(object sender, MouseButtonEventArgs e)
        {
            MessageBox.Show("Hello");
        }

thanks in advanced