a) Proceed to the API Resources page and click the button;
Decide on the Application Label, Description, your Organization name, and Website. Those are used for the application identification purposes.
Once done, click the button to create the application.
b) Once again, navigate to the API Resourses page and copy the newly created application's Secret Key;
Under the Application section, you can now see the newly created application. Copy the application's Secret Key and paste it to the Disqus Secret Key field of the Blog extension's General Settings.
c) The secret key is now set, and it's time to click the 'Admin' link to proceed to the adding a site to Disqus stage.