acosonic: Your filename should be mns2_notifications_settings. Missing an "s" in the end.
Did you restart the server when you made the filename change? Never works for me if I don't do that.
The controller is inside controllers/ and not a subfolder, right? I don't think that that changes much, but worth asking. Sorry, but from what I see it should be working. The weird part is doing GET on a edit action, that's not recommended: http://guides.rubyonrails.org/routing.html (section 3.7)