How do I add BundleConfig.cs to my project?

If you are using "MVC 5" you may not see the file, and you should follow these steps: http://www.techjunkieblog.com/2015/05/aspnet-mvc-empty-project-adding.html

If you are using "ASP.NET 5" it has stopped using "bundling and minification" instead was replaced by gulp, bower, and npm. More information see https://jeffreyfritz.com/2015/05/where-did-my-asp-net-bundles-go-in-asp-net-5/

BundleConfig is nothing more than bundle configuration moved to separate file. It used to be part of app startup code (filters, bundles, routes used to be configured in one class)

To add this file, first you need to add the Microsoft.AspNet.Web.Optimization nuget package to your web project:

Install-Package Microsoft.AspNet.Web.Optimization

Then under the App_Start folder create a new cs file called BundleConfig.cs. Here is what I have in my mine (ASP.NET MVC 5, but it should work with MVC 4):

using System.Web;
using System.Web.Optimization;

namespace CodeRepository.Web
{
    public class BundleConfig
    {
        // For more information on bundling, visit http://go.microsoft.com/fwlink/?LinkId=301862
        public static void RegisterBundles(BundleCollection bundles)
        {
            bundles.Add(new ScriptBundle("~/bundles/jquery").Include(
                        "~/Scripts/jquery-{version}.js"));

            bundles.Add(new ScriptBundle("~/bundles/jqueryval").Include(
                        "~/Scripts/jquery.validate*"));

            // Use the development version of Modernizr to develop with and learn from. Then, when you're
            // ready for production, use the build tool at http://modernizr.com to pick only the tests you need.
            bundles.Add(new ScriptBundle("~/bundles/modernizr").Include(
                        "~/Scripts/modernizr-*"));

            bundles.Add(new ScriptBundle("~/bundles/bootstrap").Include(
                      "~/Scripts/bootstrap.js",
                      "~/Scripts/respond.js"));

            bundles.Add(new StyleBundle("~/Content/css").Include(
                      "~/Content/bootstrap.css",
                      "~/Content/site.css"));
        }
    }
}

Then modify your Global.asax and add a call to RegisterBundles() in Application_Start():

using System.Web.Optimization;

protected void Application_Start()
{
    AreaRegistration.RegisterAllAreas();
    RouteConfig.RegisterRoutes(RouteTable.Routes);
    BundleConfig.RegisterBundles(BundleTable.Bundles);
}

A closely related question: How to add reference to System.Web.Optimization for MVC-3-converted-to-4 app