I\'m working on an MVC 4 project started in Visual Studio 2010. Right now I\'m working on a machine with Visual Studio 2012 as I don\'t have access to the machine I was original
I had the same problem with a VS2010 project I opened in VS2012. The Microsoft.AspNet.Web.Optimization package was missing so I just needed to download it using Nuget Package Manager.
You can run in the console: Install-Package Microsoft.AspNet.Web.Optimization