I've read a lot of documentation about FQL (awesome!) and finally I've found two great references which are suitable to retrieve all the posts from a Facebook group:
http://developers.facebook.com/docs/reference/fql/stream/
Retrieving posts and comments from a Facebook page
Well, now I have another problem: I need to give to my application the permission for retrieving the posts of a secret group (I am the owner of this group).
How can I give this permission to my application? Has the group owner (that is me in this case) to do something?
Thanks