Where is ConfigurationManager's namespace?

ConfigurationManager is a part of the System.Configuration after .Net 2.0. Add a reference to the System.Configuration dll. Try using System.Configuration.ConfigurationManager.

You need to use the System Configuration namespace, this must be included in two ways:

1. Right click the project and choose add reference, browse "Assemblies" and tick the System.Configuration assembly.

2. Include the namespace in code:

   using System.Configuration;

You need to add a reference the System.Configuration assembly. This namespace was split across a few assemblies.