If you click (+) from the exception, you should see the stack trace, post that here and will take a look.
Also, just a hint, say for example if you have a page named YourCustompage.aspx, you need this line:
Code:
<%@ Page Language="C#" AutoEventWireup="true" CodeFile="YourCustomPage.aspx.cs" Inherits="AspDotNetStorefront.YourCustomPage" MasterPageFile="~/App_Templates/Skin_1/template.master" %>
In the code behind file (.aspx.cs), make sure the Partial Class Inherits SkinBase instead of System.Web.UI.Page.
e.g.
Code:
namespace AspDotNetStorefront
{
public partial class YourCustomPage : SkinBase
{
....
....
}
}