The field(s) you want highlighting to operate on must actually be stored in the database:

<fields>
  <field name="text" type="text" indexed="true" stored="true" multiValued="true"/>
  ...

http://172.16.32.190:8080/solr/select/?q=Dexxo%0D%0A&version=2.2&start=0&rows=10&indent=on&hl=true

<lst name="highlighting">
  <lst name="product-13226">
    <arr name="text">
      <str>ELECT &lt;em&gt;DEXXO&lt;/em&gt; PRO IO</str>
    </arr>
  </lst>
  <lst name="product-13220">
    <arr name="text">
      <str>&lt;em&gt;DEXXO&lt;/em&gt; PRO IO SPR</str>
    </arr>
  </lst>
</lst>
</response>