The name ‘ConfigurationManager’ does not exist in the current context

It’s not only necessary to use the namespace System.Configuration. You have also to add the reference to the assembly System.Configuration.dll , by

1. Right-click on the References / Dependencies
2. Choose Add Reference
3. Find and add System.Configuration.

This will work for sure.
Also for the NameValueCollection you have to write:

using System.Collections.Specialized;