Tumblr: How to Allow Comments on Postsby Tricia Goss
Tumblr does not provide a native option that enables you to allow comments on your blog, but the social network does enable the installation of Disqus on many of its featured themes. Disqus is a blog comment hosting service which streamlines commenting on many different blog types.
Go to the Disqus signup page (see Resources) and register for a free account. Log in to Disqus if you already have an account.
Enter the URL of your blog into the Site URL box. Enter the site name you want to use in the Site Name box, which generates a shorter site "shortname." Add any other information requested, such as your email address. Click "Continue."
Select "Tumblr" in the Choose Your Platform list. Select the shortname provided and press "Ctrl-C" to copy it.
Navigate to the main page of your Tumblr blog and click "Customize" on the menu bar.
Scroll down to Disqus Shortname in the navigation pane. Click inside the box and press "Ctrl-V" to paste the shortname you copied. Click the "Save" button to enable Disqus comments on your Tumblr blog.