Attached you may find a sample project that demonstrates how to create and use a dummy data source from within a Web Site project. To run the project you should add references to the Telerik.Reporting.dll, Telerik.Reporting.Data.dll and Telerik.ReportView.WebForms.dll.
As you derive a class from the System.Data.DataTable you should place its code file under the App_Code folder as this the folder that ASP.NET scans for code files and compiles dynamically. Beside this it is good to rebuild your project so the Visual Studio can discover the new type and be able to load it when opening the report designer.
Here is a good place where you can learn more about the specifics of the Web Site Projects: http://msdn2.microsoft.com/en-us/library/ex526337(VS.80).aspx
The "Dummy data source" approach as mentioned before is a possible way to make the Report designer aware of the data you're binding but without specifying a real data source. This is not much related to the Telerik Reporting Designer and you're free to implement in any way you want. The solution that we offer is just one of the possible options.
In case any of you use an alternative solution, please feel free to post it here.
All the best,
the Telerik team