1. Open up /modules/portfolio/theme/current theme/main.php
around line 130 just above:
"<?php break;?>"

add:

  <!-- Comments Starts -->
  <div id="disqus_thread"></div>
  <script type="text/javascript">
    var disqus_shortname = "YOUR_DISQUS_ID_GOES_HERE";
    var disqus_identifier = "<?php echo $row->slug;?>";
    var disqus_title = "<?php echo $row->{'title' . Lang::$lang};?>";
    var disqus_url = "<?php echo Url::Portfolio("item", $row->slug);?>";
    (function() {
        var dsq = document.createElement('script'); dsq.type = 'text/javascript'; dsq.async = true;
        dsq.src = '//' + disqus_shortname + '.disqus.com/embed.js';
        (document.getElementsByTagName('head')[0] || document.getElementsByTagName('body')[0]).appendChild(dsq);
    })();
</script>
  <!-- Comments Ends -->


In the above replace YOUR_DISQUS_ID_GOES_HERE with the actual disqus id