App.config after setup

I've have a Windows Forms application that uses an app.config. It works fine when debugging in Visual Studio 2013.

In Visual Studio 2013 I use :

string database = ConfigurationSettings.AppSettings["Databasenaam"].ToString();

I create an Install Shield project and add the .exe and app.config.

After Setup on a different computer, it seems that it can’t read the app.config. If I remove all lines with ConfigurationSettings it runs fine. If I replace the lines I receive an error about a missing object reference.