The System.Runtime.Serialization.Json Namespace is in two different DLL's depending on your .net framework.
In .NET 3.5 It is in System.ServiceModel.Web.dll
In .NET 4.0 and above It is in System.Runtime.Serialization.dll.
Make sure you have added the correct DLL as a reference in your project and add using System.Runtime.Serialization.Json;
to the top of your code file.
EDIT - Consider using JSON.NET
Even though the .NET Framework supplies its own JSON Serialization and Deserialization namespaces (DataContractJsonSerializer and JavaScriptSerializer) you should investigate whether you would be better off using JSON.NET.
JSON.NET is easier to use, better performance and has far more features. http://www.newtonsoft.com/json/help/html/JsonNetVsDotNetSerializers.htm