public class StepEditModel
{

	public long Id { get; set; }

	public long TaskId { get; set; }

	[Required(), StringLength(150)]
	[DisplayName("Step Title")]
	public string Title { get; set; }

	[DataType(DataType.MultilineText)]
	public string Description { get; set; }

	[color=red]public IEnumerable<SelectListItem> TaskSelectList { get; set; }[/color]    <--- Вот здесь

}

public ActionResult Edit(long? id)
{
	if ((id == null)) {
		return new HttpStatusCodeResult(HttpStatusCode.BadRequest);
	}
	try {
		Mapper.CreateMap<Step, StepEditModel>();

		IEnumerable<Step> source = db.Steps.Find(id);

		if ((source == null)) 
                {
			return HttpNotFound();
		}

		dynamic model = Mapper.Map<Step, StepEditModel>(source);
		[color=red]model.TaskSelectList = new SelectList(db.Tasks, "Id", "Title");[/color]   < --- И вот здесь

		return View(model);
	} 
                catch (Exception ex) 
        {
		return View("Error");
	}
}