Desktop AIR app and Facebook login

I try using this lib https://code.google.com/p/facebook-actionscript-api/, but when I call FacebookDesktop.init("820024968051122",loginHandler); I get error:

code: 2500
message: An active access token must be used to query information about the current user.
type: OAuthException

If I call FacebookDesktop.init("820024968051122",loginHandler,"test_user_token"); then all work fine, but how I get token if user is not log in?