The 1.5.5 release is extremely old. Highly recommend upgrading to the current production 4.0.3 release at Oracle SQL Developer Downloads
If you must stay with 1.5.5, then something like
set serveroutput on
run in the worksheet will produce output in the Script Output tab just fine. Getting output in the DBMS Output tab, however, does not work consistently. Note the first icon in the DBMS Output toolbar is supposed to toggle serveroutput on / off. And using the Run action for a procedure or function sends the dbms_output to the Running - Log tab in the same pane as the Message tab.
Thank you so much. I now installed 4.0.3 version of sql developer & I could now enable dbms output without any issues.
Then please mark the thread ANSWERED.
I am not shown any option to mark it as answered. Could you kindly help me in this. Thanks