(I just can't get the NoteDetail node out)

<xsl:template match="TAX_CASH_FLOW_ANALYSIS/TaxCashFlowUnit/NoteDetail">
<xsl:for-each select="*/*">

                        ^^^
This selects all grand-child elements of NoteDetail, unfortunately
there are none, therefore no content is generated. You probably want
to select just "*"

<Cell ss:Index="3"><Data ss:Type="String"><xsl:value-of
select="*/."/></Data></Cell>

         ^^^^
I guess you also want a plain "." here.