directive to be used to include viewmodel in mvc4 - asp.net-mvc-4

Question

I have gone through the MVC3 tutorial for displaying the related data in asp.net page.
It has a viewmodel:
using System;
using System.Collections.Generic;
using ContosoUniversity.Models;
namespace ContosoUniversity.ViewModels
{
public class InstructorIndexData
{
public IEnumerable<Instructor> Instructors { get; set; }
public IEnumerable<Course> Courses { get; set; }
public IEnumerable<Enrollment> Enrollments { get; set; }
}
}
and a controller, with actions:
using ContosoUniversity.ViewModels;
public ActionResult Index(Int32? id, Int32? courseID)
{
var viewModel = new InstructorIndexData();
viewModel.Instructors = db.Instructors
.Include(i => i.OfficeAssignment)
.Include(i => i.Courses.Select(c => c.Department))
.OrderBy(i => i.LastName);
if (id != null)
{
ViewBag.InstructorID = id.Value;
viewModel.Courses = viewModel.Instructors.Where(i => i.InstructorID == id.Value).Single().Courses;
}
if (courseID != null)
{
ViewBag.CourseID = courseID.Value;
viewModel.Enrollments = viewModel.Courses.Where(x => x.CourseID == courseID).Single ().Enrollments;
}
return View(viewModel);
}
when i tried in MVC4 it is such that using ContosoUniversity.ViewModels; is not existing
and at
viewModel.Courses = viewModel.Instructors.Where(i => i.InstructorID == id.Value).Single().Courses;
it is getting error as cannot convert viewmodel.instructors to IEnumerable<viewmodel.Instructors>