Okay, it took me a while to see this, but there’s no way this compiles:
return String.(ConfigurationManager.AppSettings[paramName]);
You’re not even calling a method on the String type. Just do this:
return ConfigurationManager.AppSettings[paramName];
The AppSettings KeyValuePair already returns a string. If the name doesn’t exist, it will return null.
Based on your edit you have not yet added a Reference to the System.Configuration assembly for the project you’re working in.