SQL & PL/SQL

Oracle SQL Developer Version 1.2.1 does not display the output of DBMS_OUTPUT.PUT_LINE. It only display “anonymous block completed”

For example, when I run the following code in TOAD environment I get the expected output as follows:

SET serveroutput ON
BEGIN
FOR v_counter IN 1 .. 3
LOOP
DBMS_OUTPUT.PUT_LINE('v_counter = ' || v_counter);
END LOOP;
END;

Output:
v_counter = 1
v_counter = 2
v_counter = 3
PL/SQL procedure successfully completed.

Any ideas as to why (Oracle SQL Developer Version 1.2.1) is displaying “anonymous block completed” for the above code when I run the script with: Run Script (F5)?

Thanks!