OK. I am totally sure that you are missing a step there to authorize your application.

* manage_pages (or manage_groups) - This is what verifies you are a manager of a page/group and are allowed to post to it
* publish_actions - This is what allows the app (your site) to post on your behalf


The last thing to propose is to

1. Remove the application you created
2. Create a new one and
3. Follow all the steps from the begining as it apears to this video tutorial