/*
 * We need to create a class that derives from DefaultControllerFactory 
 * and override GetControllerInstance
*/
public class DependencyInjectionControllerFactory : DefaultControllerFactory
{
    public DependencyInjectionControllerFactory()
    {

       /* 
        * First thing we have to do is to initialize the Dependency
        * injection service provider (DISP) providig the type of
        * container that we want to use (Ninject, structuremap,
        * etc.) and a list "Service" instances to declare each
        * dependency.
        */
        DependencyInjectionServiceProvider.Initialize(
            IoCContainerTypeEnum.Ninject, //IoC contaner to be used
            new List<Service>() //List of dependencies
            {
                new Service() {
                    Scope = ScopeEnum.Singleton,
                    SourceType = typeof(IRepository<User>),
                    TargetType = typeof(UserRepository)
                }
            }
        );
    }

    /*
    * The default controller constructor will then use our Dependency 
    * injection service provider (DISP) to get the instance that will satisfy 
    * the dependency.
    */
    protected override IController GetControllerInstance(RequestContext context, Type controllerType)
    {
        return (IController)DependencyInjectionServiceProvider.Get(controllerType);
    }
}

Modify Global.asax.cs

protected void Application_Start()
{
    //Common ASP.NET MVC3 Application_Start() code...
    AreaRegistration.RegisterAllAreas();
    RegisterGlobalFilters(GlobalFilters.Filters);
    RegisterRoutes(RouteTable.Routes);

    //Override default Controller builder to provide dependency injection
    ControllerBuilder.Current.SetControllerFactory(new DependencyInjectionControllerFactory());
}