Thank you for providing the plugin.
The Readme.txt was not helpful.
The Readme2.txt was not helpful.
The installation instructions make sense, only in retrospect.
This widget is working nicely for me, but the instructions are not usable.
I am writing this only in case it might help someone else.
To get it to work, here's what I did.
...install the plugin as normal.
After install, you need to provide a page that displays the subscription form to the user. This is a simple form that asks for the user's email, and they need to click "OK". The install instructions had lots of words about this, which, despite their numbers, brought little clarity as to what this thing was.
The page can be implemented in any .php module, but is most often a "WordPress Page" - that is to say a page configured from the admin backend in wordpress. The Post Notification admin panel has a checkbox pertaining to this, labelled appropriately. If you check it and click "Save", then the PN plugin will create a page for you. This is probably what you want.
If you then view the page that the PN plugin created, you will see that it contains very little. It contains only template tokens, like @@post_notification_header and so on. These get replaced at runtime with the values you have configured in the PN Settings panel in the backend.
The PN plugin is driven by templates, in various languages. You select the language (==template directory) via a dropdown in the PN admin UI, then you get all those templates. The templates are used to provide the front-end user interaction. Things like the "Thank you for subscribing", and "Are you sure you want to unsubscribe?" boilerplate are provided in template files, in various languages.
I could not find a way to view or edit the templates in the PN admin UI. Instead I used a text editor to examine and edit the template files, which reside in all the various language directories.
The plugin advises that if you edit templates, be sure to save them, because an upgrade to the plugin will destroy your edits. Be advised.
In addition to editing the templates by hand, I also hacked frontend.php in the plugin, because it displayed odd things when you disable category subscriptions. I wanted a simple email notification. The plugin allows users to subscribe to some or all categories on the blog, but also has an administrative setting that eliminates that option and makes it an all-or-none subscription. When doing that the "please subscribe me" form displayed by the plugin is nonsensical. There is an ineffectual radio selection, and the words on the button is "Save", when ... it ought to be "Confirm". So I needed to hack frontend.php to make it do something reasonable.
With all that said, I recommend the plugin. It's working for me.