You need to *run* the SQL not explain it, ie
SELECT /*+ gather_plan_statistics */ e.employee_id,e.last_name,d.department_id,d.department_name
FROM hr.employees e JOIN hr.departments d
ON e.department_id = d.department_id
WHERE d.department_name LIKE 'A%';
select * from table(dbms_xplan.display_cursor(null, null, 'ALLSTATS LAST'));
Hope that helps