using Microsoft.Office.Tools.Ribbon;
using Excel=Microsoft.Office.Interop.Excel;

namespace ExcelAddIn1
{
    public partial class rbn_FKU
    {

        
        private void Ribbon1_Load(object sender, RibbonUIEventArgs e)
        {
         
        }

        private void btn_GenerateRoomNumber_Click(object sender, RibbonControlEventArgs e)
        {
           


        }
    }
}

using System.Windows.Forms;
using Microsoft.Office.Tools.Ribbon;
using Excel=Microsoft.Office.Interop.Excel;

namespace ExcelAddIn1
{
    public partial class rbn_FKU
    {

        
        private void Ribbon1_Load(object sender, RibbonUIEventArgs e)
        {
         
        }

        private void btn_GenerateRoomNumber_Click(object sender, RibbonControlEventArgs e)
        {
            Excel.Application app = new Excel.Application();
            Excel._Workbook book = app.ActiveWorkbook;
            Excel.Worksheet sheet = app.ActiveSheet;
            MessageBox.Show(string.Format("{0}",sheet.Cells[1, 1].ToString()));
        }
    }
}

private void btn_GenerateRoomsNumbers_Click(object sender, RibbonControlEventArgs e)
        {
            Excel.Application app = null;
            
                app = System.Runtime.InteropServices.Marshal.GetActiveObject("Excel.Application")
                    as Excel.Application;
           
            app.Visible = true;
            Excel.Worksheet sheet = app.ActiveSheet;
            sheet.Cells[1, 1] = "vfdgfd";


        }