<tab idMso="TabMail">
  <group id="group1" label="CRMK">
    <button
      id="MyId"
      onAction="Button_Click"
      label="Do me!"
      size="large" />
  </group>
  <group id="group2" label="group2">
    <button id="button1" label="button1" showImage="false" />
  </group>
</tab>

private void Button_Click(Object sender, RibbonControlEventArgs eventArgs)
{
  MessageBox.Show("Button clicked...");
}

public void Button_Click(Office.IRibbonControl control){ ... }