Can't find an DataSet in the App_Code

 I will make an c# ASP.NET application with an DataSet. I do these steps:
- Start Visual Studio 2008
- Add new project (ASP.NET Web Application)
- Add in my App_Data folder an SQL server database
- Add an Table in the database in the server explorer
- Add new folder with the name: App_Code
- Add new item and select DataSet
- Drag an Table from the server explorer to the DataSet
- Save the DataSet
- Open the page Default.aspx.cs
 And here I got the problem. I will add the datasat at the c# page, but I cant find it. I type "using" and don't see the name of my DataSet (TestDataSet).

When I add the DataSet in the root of the solution(and not in the folder App_Code), then I see the DataSet. But the DataSet must in the App_Code.

Here is an printscreen:



Who knows what I do wrong?

...