In the *footer* of your theme (something like `footer.ejs`) you should add *Disqus* script:

```javascript
<% if (config.disqus_shortname){ %>
<script>
  var disqus_shortname = '<%= config.disqus_shortname %>';
  <% if (page.permalink){ %>
  var disqus_url = '<%= config.url +"/"+ page.path %>';
  <% } %>
  (function(){
    var dsq = document.createElement('script');
    dsq.type = 'text/javascript';
    dsq.async = true;
    dsq.src = '//' + disqus_shortname + '.disqus.com/<% if (page.comments) { %>embed.js<% } else { %>count.js<% } %>';
    (document.getElementsByTagName('head')[0] || document.getElementsByTagName('body')[0]).appendChild(dsq);
  })();
</script>
<% } %>
```

Don't forget that Hexo is prepared to use Disqus, so you the another missing thing is to enable it in your `_config.yml`

```
# Disqus
disqus_shortname: username
```