As far as I know you do not have that option in 12c DB express.You can have Performance hub page but that is not similar to Performance tab in other editions. Install OEM 12c cloud control.
First, You cannot have Diagnostic and Tuning Pack license for any Standard Edition. It would be illegal and can put you in trouble if audit happens. You can only have diagnostic and tuning pack for Enterprise Edition. This has been always been the case.
Second, This(Feature of Performance tab) is the change they implemented in 12c DB express. It is not same as dbcontrol in other editions.