From the first search result on google:
http://msdn.microsoft.com/en-us/library/system.web.script.serialization.javascriptserializer.aspx
JavaScriptSerializer Class
Provides serialization and deserialization functionality for AJAX-enabled applications.
Inheritance Hierarchy
System.Object
System.Web.Script.Serialization.JavaScriptSerializer
Namespace: System.Web.Script.Serialization
Assembly: System.Web.Extensions (in System.Web.Extensions.dll)
So, include System.Web.Extensions.dll
as a reference.