There is no way to make the Description appear using the public object model. 

One way to solve the problem is to use the IUIElementCreationFilter interface. There is a Knowledge base article which demonstrates how:
http://devcenter.infragistics.com/Support/KnowledgeBaseArticle.aspx?ArticleID=7316

Since the article was written before the Office2007 ViewStyle was introduced, the AfterCreateChildElements method must be modified in the following manner:
public void AfterCreateChildElements(Infragistics.Win.UIElement parent)
{
    //  If the parent is an EmbeddableUIElementBase, get the associated AppointmentUIElement
    //  and remove all elements except for the EmbeddableUIElementBase, and change the bounds
    //  of the EmbeddableUIElementBase to occupy the entire bounds of the AppointmentUIElement.
    EmbeddableUIElementBase embeddableElement = parent as EmbeddableUIElementBase;
    AppointmentUIElement appointmentElement = embeddableElement != null ? embeddableElement.GetAncestor( typeof(AppointmentUIElement) ) as AppointmentUIElement : null;
if ( appointmentElement != null )
    {
        //  Get the associated appointment, and the text that should be displayed for it.
        Appointment appointment = appointmentElement.GetContext( typeof(Appointment) ) as Appointment;
        string text = this.GetAppointmentText( appointment );

        //  Set the bounds of the embeddable element
        embeddableElement.Rect = appointmentElement.RectInsideBorders;

        //  Get the embeddable element's TextUIElement
        TextUIElement textElement = embeddableElement.GetDescendant( typeof(TextUIElement) ) as TextUIElement;
       
        if ( textElement != null )
        {
            //  Change the text displayed by the embeddable element based on
            //  the DisplayStyle.
            textElement.Text = text;

            //  Reverse iterate the AppointmentUIElement's ChildElements collection
            //  and remove all elements except for the EmbeddableUIElementBase
            UIElementsCollection childElements = appointmentElement.ChildElements;
            int childElementCount = childElements.Count;

            for ( int i = childElementCount - 1; i >= 0; i -- )
            {
                UIElement element = childElements[i];
                if ( element == embeddableElement )
                    continue;

                childElements.Remove( element );
            }
        }
    }
}