Activate the plugin from the control panel and restart NodeBB.
You should see a Imgur menu item in the control panel. Paste the Client ID to the "Imgur Client ID" in the plugin page. Save and you should be able to upload images by dragging them into the composer window.
No documentation for this yet! See `the plugin thread <https://community.nodebb.org/topic/796/nodebb-plugin-s3-uploads-store-your-uploads-in-aws-s3>`_ for more information.