The key here is to add
<add namespace="System.Web.Optimization" />
to BOTH web.config files. My scenario was that I had System.Web.Optimization reference in both project and the main/root web.config but @Scripts still didn’t work properly. You need to add the namespace reference to the Views web.config file to make it work.
UPDATE:
Since the release of MVC 4 System.Web.Optimization is now obsolete. If you’re starting with a blank solution you will need to install the following nuget package:
Install-Package Microsoft.AspNet.Web.Optimization
You will still need to reference System.Web.Optimization in your web.config files. For more information see this topic:
How to add reference to System.Web.Optimization for MVC-3-converted-to-4 app
As many pointed out, restart of VS could be required after the above steps to make this work.