sorry for late reply... I just saw the PW....
Any way if you are coming from Lahore side... Leave the highway at Multanby Pass and continue to Chowk Kumharan... From Chowk Kumharan Take the Masoom Shah Road upto Doulat Gate... Then Hussain Agahi.. From Hussain Agahi take a little right turn leave the main road... and get upto the Fort... and then turn left and continue upto parking at other end of Qila..
If you coming from Bahawalpur side... Better to come to Multan General Bus Stand and then continue to chowk Kumharan and then use above route...