To bring your old comments into Disqus, they must be exported from Wordpress and imported into Disqus. There are two ways to do this:
Use the Export Comments button under the Disqus plugin's Advanced Options tab. This will automate the export and import process.
If the export comments function isn't working on your site, you have the alternate option of exporting your entire site from WordPress and importing manually. This can be done with the following steps:
Deactivate all plugins except Disqus at your WP Dashboard > Plugins page.
Export your WordPress site in WXR format at your WP Dashboard > Tools > Export.
Note: Sometimes the WXR file cuts off prematurely, resulting in not all comments being exported. This can be caused by Wordpress not allocating enough resources. In that case, we recommend exporting your blog in date segments to keep the file size down.
Import your exported WXR file.