Does the site load when accessing it from your own browser (or better yet, from a different machine connected to the Internet)? Are you pasting in an
http://127.0.0.1:8000 address? If so, that won't work with tools like
responsinator.com. Your site will need to be publicly accessible on the Internet before that service can work for you. If you are running the Django development server (runserver), then your site is only available to you on your local machine by default (unless you pass in extra options and do other funny things with firewalls, etc.)
I use the Web Developer toolbar for Firefox. It includes the ability to view your pages in multiple pane sizes to test responsiveness (pretty close to how responsinator displays it), as well as many other useful tools. It also works just dandy using the Django runserver, since I'm developing and running the development server on the same machine where I'm doing my initial responsive testing. Highly recommended.