You need to declare the source document's namespace in the <stylesheet>
element, assign it a prefix and use that prefix when addressing the source document's elements. So your stylesheet's root element would look like this:
<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:ns="http://localhost/elegibilidad">
(note that it's not necessary to declare the Filemaker namespace here), and your calls to get the data need to be changed to:
<COL><DATA><xsl:value-of select="//ns:Cabecera/ns:CodigoREEEmpresaEmisora"/></DATA></COL>
<COL><DATA><xsl:value-of select="//ns:Medidas/ns:CodUnificadoPuntoSuministro"/></DATA></COL>