I have read the other threads about how the FB requests are only for Canvas apps, but maybe these answer are a bit old and I want to be sure this is current information, because on this page http://developers.facebook.com/docs/channels/ FB says:
If you have built a Canvas or Mobile Web application, Requests are a great way to enable users to invite their friends.
Difference between Canvas & mobile: Facebook canvas app vs Facebook mobile web app
Then, are requests just for Canvas even if they say that?
Answering my own question:
You need a canvas url in the app settings to have access to requests. These requests can be done on your mobile web app (not just the the canvas).
If you just want a mobile app and not a canvas app but still use request, fill the canvas url on the app panel and on the pageof this url use a javascript redirection to your mobile app.