private void ThisAddIn_Startup(object sender, System.EventArgs e)
{
    CommandBar cellbar = this.Application.CommandBars["Cell"];
    CommandBarButton button = (CommandBarButton) cellbar.FindControl(MsoControlType.msoControlButton, 0, "MYRIGHTCLICKMENU", Missing.Value, Missing.Value);
    if (button == null)
    {
        // add the button
        button = (CommandBarButton) cellbar.Controls.Add(MsoControlType.msoControlButton, Missing.Value, Missing.Value, cellbar.Controls.Count, true);
        button.Caption = "Refresh";
        button.BeginGroup = true;
        button.Tag = "MYRIGHTCLICKMENU";
        button.Click += new _CommandBarButtonEvents_ClickEventHandler(MyButton_Click);
    }
}

private void MyButton_Click(CommandBarButton cmdBarbutton, ref bool cancel)
{
    System.Windows.Forms.MessageBox.Show("MyButton was Clicked", "MyCOMAddin");
}

//reset commandbars
Application.CommandBars["Cell"].Reset();