This is a pretty genius way to get members of a messageboard/site to use your service.
One minor critique: the instructions say "Once you have subscribed to HN alerts, go to the Hacker News Service Settings page and make sure that Send Email Notifications is checked", but I do not see any "Send Email Notifications" checkbox on the page linked to.
Have you enabled Receive Email Notifications under the Advanced Settings? http://notifo.com/user/settings/advanced - this will display the email checkboxes on the service settings pages.
One minor critique: the instructions say "Once you have subscribed to HN alerts, go to the Hacker News Service Settings page and make sure that Send Email Notifications is checked", but I do not see any "Send Email Notifications" checkbox on the page linked to.