<ribbon>
    <tabs>
      <tab id="testTub" label="test tub" >
        <group id="tstGroup" label="test group" >
          <button id="usedRange" visible="true" size="large" 
                  label="test func" keytip="C" 
                  screentip="push me" 
                  onAction="customUI.xlsm!Module1.foo" image="usedRange"/>
        </group>
      </tab>
    </tabs>
  </ribbon>

Public Sub foo(control As IRibbonControl, Optional j As Integer)
    MsgBox "hi there " & j
End Sub