Referencing Content From Master Page?

I have a Master Page that contains a Menu Control. Each menu item clicked is supposed to load a new aspx page. My problem is (since I'm new to Master Pages) actually loading a Content Page. This is what my MP code looks like and it's only just a snippet. The first set of code are the ContentPlaceHolders in my MP. The second set is the code in my code-behind file in my MP. Hope this makes sense. In the VB code, all I'm trying to do is load my Customer.aspx page. What am I missing?

HTML Code:
<asp:ContentPlaceholder ID="Welcome" runat="server"></asp:ContentPlaceholder>
<asp:ContentPlaceHolder ID="MainArea" runat="server"></asp:ContentPlaceHolder>
<asp:ContentPlaceHolder ID="CustContent" runat="server"></asp:ContentPlaceHolder>

Code:
Private Sub Menu1_MenuItemClick(ByVal sender As Object, ByVal e As System.Web.UI.WebControls.MenuEventArgs) Handles Menu1.MenuItemClick
Try
Select Case e.Item.Value
Case "mnuCustomerMaintenance"
MainArea.Page = Customer
End Select
Catch ex As Exception
End Try
End Sub