tha_watcha, I really appreciate your help! Thank you very much! You were right and now the system can open a box to choose between:
"Login with an user Account that is connected with Facebook. Or if no account is found a new account will be automatically created in the forums."
"Connecting an existing forums account with your facebook".
But there is something still wrong because even with a user correctly connected on Facebook the message below is showed:
"The facebook login failed, User does not exists, or the account is not connected".
Can you imagine why this is happening?