Hi mangeshd29,
System.Web.Script.Serialization namespace is available in the assembly System.Web.Extensions.dll
So you need to add the reference to System.Web.Extensions.dll in your project to use the System.Web.Script.Serialization namespace.
It should be listed in the .NET tab of the Add Reference dialog.
If not found you can found the System.Web.Extensions.dll in the below path.
On 64 bit OS
C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.0\System.Web.Extensions.dll
On 32 bit OS
C:\Program Files\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.0\System.Web.Extensions.dll
Change the Microsoft .NET Framework version if required.
