How to add reference System.Web.Optimization

The Microsoft.Web.Optimization package is now obsolete. With the final release of ASP.NET (MVC) 4 you should install the Microsoft ASP.NET Web Optimization Framework:

Install the package from nuget:

Right click on References, Select Manage Nuget Packages. Type Microsoft.AspNet.Web.Optimization in search box and press enter you will get the package. Click on install.

Create and configure bundle(s) in App_Start\BundleConfig.cs:

public class BundleConfig
{
    public static void RegisterBundles(BundleCollection bundles) {
        bundles.Add(new ScriptBundle("~/Scripts/jquery").Include(
            "~/Scripts/Lib/jquery/jquery-{version}.js",
            "~/Scripts/Lib/jquery/jquery.*",
            "~/Scripts/Lib/jquery/jquery-ui-{version}.js")
        );

        bundles.Add(new ScriptBundle("~/Scripts/knockout").Include(
             "~/Scripts/Lib/knockout/knockout-{version}.js",
             "~/Scripts/Lib/knockout/knockout-deferred-updates.js")
        );
    }
}

Call the RegisterBundles() function from Application_Start() in your global.asax.cs:

using System.Web.Optimization;

protected void Application_Start() {
     ...
     BundleConfig.RegisterBundles(BundleTable.Bundles);
     ...
}

In your view.cshtml include the Optimization namespace and render the bundle(s):

@using System.Web.Optimization

@Scripts.Render("~/Scripts/jquery")
@Scripts.Render("~/Scripts/knockout")