Here at least your customization file name is wrong. You are also missing whitespace before the onAction attribute. Groups don't have images associated with them.
You are aware that your code is going to destroy any existing ribbon or QAT customizations. Correct?
Code:
hFile = FreeFile
User = Environ("HomePath")
path = "C:" & User & "\AppData\Local\Microsoft\Office\"
fileName = "Word.officeUI" '*** This is the filename that works here.
ribbonXml = "<mso:customUI xmlns:mso='http://schemas.microsoft.com/office/2009/07/customui'>" & vbNewLine
ribbonXml = ribbonXml + " <mso:ribbon>" & vbNewLine
ribbonXml = ribbonXml + " <mso:qat/>" & vbNewLine
ribbonXml = ribbonXml + " <mso:tabs>" & vbNewLine
ribbonXml = ribbonXml + " <mso:tab id='mso_c1.109B239D' label='Formatting' insertBeforeQ='mso:TabDeveloper'>" & vbNewLine
ribbonXml = ribbonXml + " <mso:group id='mso_c2.109B239D' label='Export' autoScale='true'>" & vbNewLine
ribbonXml = ribbonXml + " <mso:button id='x1:startExport_0_109F2716' " & vbNewLine
ribbonXml = ribbonXml + "imageMso='ViewFullScreenView' onAction='startExport' visible='true'/>" & vbNewLine
ribbonXml = ribbonXml + " </mso:group>"
ribbonXml = ribbonXml + " </mso:tab>" & vbNewLine
ribbonXml = ribbonXml + " </mso:tabs>" & vbNewLine
ribbonXml = ribbonXml + " </mso:ribbon>" & vbNewLine
ribbonXml = ribbonXml + "</mso:customUI>"
Open path & fileName For Output Access Write As hFile
Print #hFile, ribbonXml
Close hFile