Let's define the page in the template for [s:]slides:

<xsl:template match="s:slides">
<fo:root font-size="18pt">
   <fo:layout-master-set>
      <fo:simple-page-master master-name="slide" page-height="8.5in" page-width="11in">
         <fo:region-body margin-top="0.5in" margin-bottom="0.5in" 
                         margin-left="0.75in" margin-right="0.75in"/>
      </fo:simple-page-master>
   </fo:layout-master-set>
   <fo:page-sequence master-reference="slide">
      <fo:title><xsl:value-of select="s:title/text()"/></fo:title>
      <fo:flow flow-name="xsl-region-body">
         <xsl:apply-templates/>
      </fo:flow>
   </fo:page-sequence>
</fo:root>
</xsl:template>

The contents of the document will flow into the region-body of the pages.