SQL Developer Data Modeler

I am using SDDM 4.0.1.836.  I am getting the error "Wrapped java.lang.ClassNotFoundException: oracle.jdbc.OracleDriver(Query Test#3)".

importPackage(java.sql);
java.lang.Class.forName ("oracle.jdbc.OracleDriver");
conn = java.sql.DriverManager.getConnection ("jdbc:oracle:thin:@localhost:1521:DESN", "READONLY_USER", "READONLY");
stmt = conn.createStatement();
rset = stmt.executeQuery("SELECT * FROM DUAL");
rset.close();
stmt.close();
conn.close();

There is an archived thread,  , but this issue was not resolved.