CS0246: The type or namespace name 'DataSet' could not be found

[mariocatch]

This means the type (in this case, DataSet) can not be found within the imported namespaces for your namespace).

If you're using Visual Studio IDE, right click on the DataSet type in your IDE. Go to resolve, and select either option there (add using or implement explicitly).

This will either add the namespace up top in your using statements, or it will add it explicitly to the DataSet usage in-code for you.