The ViewModelBase is a class that I created in the sample project used in the video. The part you are intested in is the support for INotifyPropertyChanged. Here is the code for the file:

using System.Collections.Generic;
using System.ComponentModel;

namespace Silverlight92LOB.Views
{
    public class ViewModelBase : INotifyPropertyChanged
    {
        #region INotifyPropertyChanged Support
        public event PropertyChangedEventHandler PropertyChanged;

        protected virtual void RaisePropertyChangedEvent(params string[] propertyNames)
        {
            if (this.PropertyChanged != null)
            {
                foreach (string name in propertyNames)
                {
                    this.PropertyChanged(this, new PropertyChangedEventArgs(name));
                }
            }
        }
        #endregion
    }
}

that video is what I'm trying to work along with. I apparently am missing a reference to the ViewModel base.

wehre is it?