[System.Runtime.InteropServices.ProgId("ExcelCOMAddin.MyConnect")]
[System.Runtime.InteropServices.ComDefaultInterface(typeof(IMyConnect))]
// [System.Runtime.InteropServices.Guid("DE3911A1-C82F-42F1-BEFB-F3802E1B6D34")]
[System.Runtime.InteropServices.Guid("C33CAD6D-E14D-45D5-94E5-6A9861061129")]
[System.Runtime.InteropServices.ClassInterface(
    System.Runtime.InteropServices.ClassInterfaceType.None)]
[System.Runtime.InteropServices.ComVisible(true)]
public class MyConnect : IMyConnect
{
    Microsoft.Office.Interop.OneNote.Application _ApplicationObject;
    Microsoft.Office.Core.COMAddIn _AddinInstance;

    public void OnConnection(
        object Application,
        Extensibility.ext_ConnectMode ConnectMode,
        object AddInInst,
        ref Array custom)
    {
        this._ApplicationObject = (Microsoft.Office.Interop.OneNote.Application)Application;

        if (this._AddinInstance == null)
        {
            this._AddinInstance = (Microsoft.Office.Core.COMAddIn)AddInInst;
            this._AddinInstance.Object = this;
        }

        System.Windows.Forms.MessageBox.Show("onConnection");
    }
    public void OnDisconnection(
        Extensibility.ext_DisconnectMode RemoveMode,
        ref Array custom)
    {
        this._ApplicationObject = null;
        this._AddinInstance = null;
    }
    public void OnAddInsUpdate(ref Array custom)
    { }
    public void OnStartupComplete(ref Array custom)
    { }
    public void OnBeginShutdown(ref Array custom)
    { }

    //public string GetCustomUI(string RibbonID)
    //{
    //    return "";
    //}


    public string GetCustomUI(string RibbonID)
    {
        return GetResourceText("COMExcelModelCover.Ribbon.xml");
    }

    private static string GetResourceText(string resourceName)
    {
        System.Reflection.Assembly asm = System.Reflection.Assembly.GetExecutingAssembly();
        string[] resourceNames = asm.GetManifestResourceNames();

        for (int i = 0; i < resourceNames.Length; ++i)
        {
            if (string.Compare(resourceName, resourceNames[i], StringComparison.OrdinalIgnoreCase) == 0)
            {
                using (System.IO.StreamReader resourceReader = new System.IO.StreamReader(asm.GetManifestResourceStream(resourceNames[i])))
                {
                    if (resourceReader != null)
                    {
                        return resourceReader.ReadToEnd();
                    }
                }
            }
        }
        return null;
    }

    public void Button_One_Click(Microsoft.Office.Core.IRibbonControl control)
    {
        System.Windows.Forms.MessageBox.Show("Button_One_Clicked");
    }

    public void Button_Two_Click(Microsoft.Office.Core.IRibbonControl control)
    {
        System.Windows.Forms.MessageBox.Show("Button_Two_Clicked");
    }


}`

<?xml version="1.0" encoding="utf-8" ?>
<customUI xmlns="http://schemas.microsoft.com/office/2006/01/customui">
	<ribbon>
		<tabs>
			<tab id="MyNewRibbonTab"
					  insertBeforeMso="TabInsert"
					  label="BETTER">
				<group id="Group_ID_One" label="MyGroup">
					<button id="Button_ID_One"
								  imageMso="Help"
								  onAction="Button_One_Click"
								  label="Button 1"
								  size="large"/>
					<separator id="Separator_ID_One"/>
					<button id="Button_ID_One"
								  imageMso="Help"
								  onAction="Button_One_Click"
								  label="Button 1"
								  size="large"/>
				</group>
			</tab>
		</tabs>
	</ribbon>
</customUI>