Facebook Page Publish publishes your posts to a facebook page. To do so, it has to connect to facebook. This is done via a facebook application.
You have to create your own facebook application: Create application
Go to your application's settings page and copy the application ID and the secret to the approptiate option fields of the plugin.
To authorize your blog to be able to use your application, you have to fill in the URL and optionally the domain of your blog:
Go to your facebook page (or create one) where your blog post shall appear. Copy the page ID to the appropriate option field of the plugin:
Confirm the plugin settings and don't forget to authorize your blog to access your facebook page. To do so, simply click the authorize button on the plugin's settings page:
Further questions? Look here!