I'm getting the "missing a using directive or assembly reference" and no clue what's going wrong

You probably don't have the System.Configuration dll added to the project references. It is not there by default, and you have to add it manually.

Right-click on the References and search for System.Configuration in the .net assemblies.

Check to see if it is in your references...

Right-click and select Add Reference...

Find System.Configuration in the list of .Net Assemblies, select it, and click Ok...

The assembly should now appear in your references...

.Net framework of the referencing dll should be same as the .Net framework version of the Project in which dll is referred