How does NerdDinner's AddModelErrors work?

I'm going through the NerDinner free tutorial http://nerddinnerbook.s3.amazonaws.com/Intro.htm

I got to somewhere in Step 5 where it says to make the code cleaner we can create an extension method. I look at the completed code and it has this to use the extension method:

catch
{
    ModelState.AddModelErrors(dinner.GetRuleViolations());
    return View(new DinnerFormViewModel(dinner));
}

And then this as the extension method's definition.

namespace NerdDinner.Helpers {

    public static class ModelStateHelpers {

        public static void AddModelErrors(this ModelStateDictionary modelState, IEnumerable<RuleViolation> errors) {

            foreach (RuleViolation issue in errors) {
                modelState.AddModelError(issue.PropertyName, issue.ErrorMessage);
            }
        }
    }
}

I try to follow what the tutorial says combined with what the code contains but receive the expected error that there is no AddModelErrors method that accepts only 1 argument.

I'm obviously missing something very important here. What is it?

You need to include the helpers reference;

using NerdDinner.Helpers;

and

using NerdDinner.Models;

Then check for valid and add the errors;

if (!dinner.IsValid)
{
    ModelState.AddModelErrors(dinner.GetRuleViolations());
    return View(dinner);
}

You must also have a partial class for your dinner;

public partial class Dinner
{
    public bool IsValid
    {
        get { return (GetRuleViolations().Count() == 0); }
    }

    public IEnumerable<RuleViolation> GetRuleViolations()
    {
        if (String.IsNullOrEmpty( SomeField ))
            yield return new RuleViolation("Field value text is required", "SomeField");
    }

    partial void OnValidate(ChangeAction action)
    {
        if (!IsValid)
            throw new ApplicationException("Rule violations prevent saving");
    }
}

Don't forget the RuleViolation class;

public class RuleViolation
{
    public string ErrorMessage { get; private set; }
    public string PropertyName { get; private set; }

    public RuleViolation(string errorMessage)
    {
        ErrorMessage = errorMessage;
    }

    public RuleViolation(string errorMessage, string propertyName)
    {
        ErrorMessage = errorMessage;
        PropertyName = propertyName;
    }
}