To get disqus shortname go to disqus.com website
Login to your account if you have one, If you don't have disqus account then signup.
Ater completing the signup you will be redirected to your profile page and click on Add Disqus to Site
You will get this screen
Click on Start Using Engage to go to the Site Profile
Add your details and then remember the Shortname to add it in the Comments Setting