For displaying Disqus on your site, typically on individual article or post pages.

1. In Tumblr, visit Settings > Edit theme
2. Select "Edit HTML".
3. Copy and paste the following code anywhere between the <head> and </head> tags: <meta name='text:Disqus Shortname' content='' />
4. Copy and paste the following code immediately after {/block:Posts}:

{block:IfDisqusShortname} 

    /* * * CONFIGURATION VARIABLES: EDIT BEFORE PASTING INTO YOUR WEBPAGE * * */
    var disqus_shortname = '{text:Disqus Shortname}'; // Required - Enter shortname in Tumblr Theme Options
    var disqus_url = '{Permalink}';     /* * * DON'T EDIT BELOW THIS LINE * * */
    (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);
    })();

Please enable JavaScript to view the comments powered by Disqus.
blog comments powered by Disqus{/block:IfDisqusShortname}

5. Select "Update Preview" and then "Save"
6. Go back to "Edit Theme"
7. Enter your Disqus site shortname in the "Disqus Shortname" field. If you don't, Disqus won't work.

Comment count code

For displaying comment counts on your site's home page.

In Tumblr, visit Settings > Your Site > Customize
Select "Edit HTML".
Copy and paste the following code immediately before {/block:Posts}:

{block:IndexPage}Comments

    var disqus_shortname = '{text:Disqus Shortname}';    (function () {
        var s = document.createElement('script'); s.async = true;
        s.type = 'text/javascript';
        s.src = '//' + disqus_shortname + '.disqus.com/count.js';
        (document.getElementsByTagName('HEAD')[0] || document.getElementsByTagName('BODY')[0]).appendChild(s);
    }());

{/block:IndexPage}