master.xml

<current-game-rules>
    <section id="9" handle="rules">Rules</section>
    <entry id="12">
      <rule-brief handle="eat-a-serving-of-fruits-or-veggies">Eat fruits or veggies</rule-brief>
      <points handle="1">1</points>
      <game-link>
        <item id="9" handle="2013-01-12-19-04-00" section-handle="games" section-name="Games">2013-01-12 19:04:00</item>
      </game-link>
      <date-created iso="2013-01-12T19:07:00-06:00" time="19:07" weekday="6" offset="-0600">2013-01-12</date-created>
    </entry>
    <entry id="11">
      <rule-brief handle="exercise-for-30-minutes">Exercise</rule-brief>
      <points handle="2">2</points>
      <game-link>
        <item id="9" handle="2013-01-12-19-04-00" section-handle="games" section-name="Games">2013-01-12 19:04:00</item>
      </game-link>
      <date-created iso="2013-01-12T19:06:00-06:00" time="19:06" weekday="6" offset="-0600">2013-01-12</date-created>
    </entry>
  </current-game-rules>
  <member-activity>
    <section id="10" handle="activity">Activity</section>
    <entry id="84">
      <member-link>
        <item id="7" handle="jesse" section-handle="members" section-name="Members">Jesse</item>
      </member-link>
      <game-link>
        <item id="9" handle="2013-01-12-19-04-00" section-handle="games" section-name="Games">2013-01-12 19:04:00</item>
      </game-link>
      <rule-link>
        <item id="12" handle="eat-a-serving-of-fruits-or-veggies" section-handle="rules" section-name="Rules">Eat a serving of fruits or veggies</item>
      </rule-link>
      <date-created iso="2013-01-16T00:00:00-06:00" time="00:00" weekday="3" offset="-0600">2013-01-16</date-created>
    </entry>
    <entry id="81">
      <member-link>
        <item id="7" handle="jesse" section-handle="members" section-name="Members">Jesse</item>
      </member-link>
      <game-link>
        <item id="9" handle="2013-01-12-19-04-00" section-handle="games" section-name="Games">2013-01-12 19:04:00</item>
      </game-link>
      <rule-link>
        <item id="11" handle="exercise-for-30-minutes" section-handle="rules" section-name="Rules">Exercise for 30 minutes</item>
      </rule-link>
      <date-created iso="2013-01-16T00:00:00-06:00" time="00:00" weekday="3" offset="-0600">2013-01-16</date-created>
    </entry>
    <entry id="79">
      <member-link>
        <item id="7" handle="jesse" section-handle="members" section-name="Members">Jesse</item>
      </member-link>
      <game-link>
        <item id="9" handle="2013-01-12-19-04-00" section-handle="games" section-name="Games">2013-01-12 19:04:00</item>
      </game-link>
      <rule-link>
        <item id="12" handle="eat-a-serving-of-fruits-or-veggies" section-handle="rules" section-name="Rules">Eat a serving of fruits or veggies</item>
      </rule-link>
      <date-created iso="2013-01-16T00:00:00-06:00" time="00:00" weekday="3" offset="-0600">2013-01-16</date-created>
    </entry>
  </member-activity>

master.xsl

<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0">

  <xsl:output method="xml" indent="yes" />

  <xsl:template match="/">
    <xsl:variable name="totalscore" select="0"/>
    <xsl:for-each select="/member-activity/entry">
      <xsl:variable name="ruleid" select="rule-link/item/@id"/>
      <xsl:variable name="worth" select="//current-game-rules/entry[@id = $ruleid]/points"/>
      <xsl:variable name="totalscore" select="$totalscore + $worth"/>
    </xsl:for-each>
    <p><xsl:text>Total Score: </xsl:text><xsl:value-of select="$totalscore"/></p>
  </xsl:template>

</xsl:stylesheet>