Run a query in SQL Server Management studio like this:

SELECT * FROM View_<NameSpace>_TestDocType_Joined

and see what you get...by default Kentico creates a view for you that has all document type and node information in it. If you look at the end of the view results, you'll see your custom document type columns.

Where <NameSpace> is replace that with your namespace value in the General tab of your custom document type.