1. I see you have an idMso= attribute in your tab tag, which refers to ids of the built-in tabs - you should probably use id=.
2. And, you can't say size='Large' , you need size='large' . The mixed case makes the whole Ribbon not load. How irritating is that?
(I see that you are using the Excel 2010 schema in your example. This Ribbon will only show under Excel 2010. For a Ribbon that would show under both Excel 2007 and Excel 2010 you'd put
This works for me on Excel 2007:
<tab id='Tab1' label='EManager FR 4'>
<group id='Groupx1' label='Log'>
<button id='EFRViewLogFiles' label='View Files' getImage='GetRibbonControlImage' size='large' onAction='OnButtonPressed'/>