PL-SQL logging info with DBMS_OUTPUT.PUT_LINE in SQL Developer

unless you explicitly enable the logging, you will never see anything...

SET SERVEROUTPUT ON;

BEGIN

  DBMS_OUTPUT.ENABLE;
 DBMS_OUTPUT.PUT_LINE('Hello...');

 DBMS_OUTPUT.PUT_LINE('Goodbye.');
 END;
 /

in a stored procedure, just do

dbms_output.enable (1000000);