To use Disqus comments on your Magento/WordPress integrated blog, you first need to install the Disqus Comments plugin in WordPress, which is a free plugin. Next, configure your Disqus account via the WordPress Admin. You can use this to import any existing comments in WordPress into Disqus.
To get your Disqus comments to show on your integrated blog (in place of your default WordPress comments), you will need the FishPig Disqus extension for Magento. This extension takes your Disqus account details from WordPress and displays the necessary code to display the comments in your Magento blog. The extension will do this automatically and you won't need to configure anything in Magento for this to work.
http://fishpig.co.uk/magento/wordpress-integration/disqus/
If after installing the Disqus comments extension the comments do not display, the first thing to check is that you are using the latest version of Magento WordPress Integration. If you aren't, upgrade to the latest version.
If this doesn't fix the problem, check whether you have the wordpress.xml layout file or any wordpress templates in your custom theme. If you do, rename them so that the new version (in base/default) is used. You may need to refresh your Magento caches after doing this.