You must gather system statistics with dbms_stats.gather_system_stats during a typical load on your system. You can simply run the following:

SQL> exec dbms_stats.gather_system_stats('START');

After an hour or several hours run:

SQL> exec dbms_stats.gather_system_stats('STOP');

Your system stats will be stored in sys.aux_stats$. Check the values in this table whether the value for MREADTIM is bigger than SREADTIM. If this is not the case than system statistics will not be used.