you need a copy of the C# wrapper RestClient.cs or the DLL RestClient.dll, available from here
You should place the file in your class path or in the folder where you are executing the code.
You must use the RestClient assembly in your C# code:

using RestAPI;

Microsoft.CSharp
System
System.Core
System.Data
System.Data.SetExtensions
System.Web
System.Xml
System.Xml.Linq