Trouble with vs 2008 excel add in walk through

namespace FirstExcelAddIn
{
    public partial class ThisAddIn
    {
        private void ThisAddIn_Startup(object sender, System.EventArgs e)
        {
            this.Application.WorkbookBeforeSave += new Microsoft.Office.Interop.Excel.AppEvents_WorkbookBeforeSaveEventHandler(Application_WorkbookBeforeSave);
            
        }

        private void ThisAddIn_Shutdown(object sender, System.EventArgs e)
        {
        }

        #region VSTO generated code

        /// <summary>
        /// Required method for Designer support - do not modify
        /// the contents of this method with the code editor.
        /// </summary>
        private void InternalStartup()
        {
            this.Startup += new System.EventHandler(ThisAddIn_Startup);
            this.Shutdown += new System.EventHandler(ThisAddIn_Shutdown);
        }
        
        #endregion
        void Application_WorkbookBeforeSave(Microsoft.Office.Interop.Excel.Workbook Wb, bool SaveAsUI, ref bool Cancel)
        {
            Excel.Worksheet activeWorksheet = ((Excel.Worksheet)Application.ActiveSheet);
            Excel.Range firstRow = activeWorksheet.get_Range("A1", missing);
            firstRow.EntireRow.Insert(Excel.XlInsertShiftDirection.xlShiftDown, System.Type.Missing);
            Excel.Range newFirstRow = activeWorksheet.get_Range("A1", missing);
            newFirstRow.Value2 = "This text was added by using code";
        }

    }
}

throw new System.Exception("An exception has occurred.");