Since you are using SQL Developer, you have a couple of options.
In SQL Developer, go to
View | DBMS Output to ensure that the DBMS Output window is visible. In the DBMS Output window, choose the "plus" icon and select the connection that you want to write data to the DBMS Output window. Then run the PL/SQL block in the SQL Worksheet window using the right arrow (Ctrl+Enter in Windows). You'll see the output appear in the DBMS Output window.
Alternately, you can put both the SQL*Plus
SET SERVEROUTPUT ON command and the PL/SQL block in the SQL Worksheet and run it as a script (F5 in Windows). That will display the output immediately below the "anonymous block completed" message in the Script Output window.