Hallo,
I tried to follow A Guide to Customizing the Office 2007 Ribbon at TechNet but wihtout the Macros, that why I use a XLSX instead of XLSM (tried it as templates as well). Unfortunately the ribbon doesn't show up. I took a very simple XML markup from MSDN:
Ribbon: customUI.xml
<?xml version="1.0" encoding="utf-8"?><customUI xmlns="http://schemas.microsoft.com/office/2009/07/customui"><ribbon startFromScratch="false"><tabs><tab idMso="TabCreate" visible="false" /><tab id="dbCustomTab" label="A Custom Tab" visible="true"><group id="dbCustomGroup" label="A Custom Group"><control idMso="Paste" label="Built-in Paste" enabled="true"/></group></tab></tabs></ribbon></customUI>
.rels
<?xml version="1.0" encoding="UTF-8" standalone="yes"?><Relationships xmlns="http://schemas.openxmlformats.org/package/2006/relationships"><Relationship Id="rId3" Type="http://schemas.openxmlformats.org/officeDocument/2006/relationships/extended-properties" Target="docProps/app.xml"/><Relationship Id="rId2" Type="http://schemas.openxmlformats.org/package/2006/relationships/metadata/core-properties" Target="docProps/core.xml"/><Relationship Id="rId1" Type="http://schemas.openxmlformats.org/officeDocument/2006/relationships/officeDocument" Target="xl/workbook.xml"/><Relationship Id="rId4" Type="http://schemas.openxmlformats.org/officeDocument/2006/relationships/custom-properties" Target="docProps/custom.xml"/><Relationship Id="rId5" Type="http://schemas.microsoft.com/office/2009/relationships/ui/extensibility" Target="customUI/customUI.xml"/></Relationships>
to only change what was neccessary is to adapt
<Relationship Id="rId5" Type="http://schemas.microsoft.com/office/2006/relationships/ui/extensibility" Target="customUI/customUI.xml"/><customUI xmlns="http://schemas.microsoft.com/office/2006/01/customui">
to your current office version
...Type="http://schemas.microsoft.com/office/2009/relationships/ui/extensibility" Target="customUI/customUI.xml"/>
...schemas.microsoft.com/office/2009/07/customui">