At a guess the problem relates to not having your web.config setup correctly for handling c#.

Since DNN is a .vb based application by default - you need to make sure you tell the runtime system to recognise and process you c# App_Code business rules correctly.

In your web.config file look for the following section:

      <!-- register your app_code subfolders to generate granular assemblies during compilation
      <codeSubDirectories>
         <add directoryName="sub-directory name"/>
      </codeSubDirectories>

You will need to add an   <add directoryName="sub-directory name"/>  entry for each folder in \App_Code that contains c# based business rules.

And remove the COMMENT around the whole codeSubDirectories xml block as well.

Westa