To get there from Islamabad/Rawalpindi, drive east on the Grand Trunk (GT) Road. Appox. 100 km (1.5 hour drive) you will reach Dina. Past Dina, make a U-turn, and a sign on the left (next to a police station) will direct you towards Rohtas Fort, 7km from GT Road. The drive will take you trough the heartland of Jehlum district, surrounded by farm land and villages. Once you cross the newly constructed brigde on Kahan River, you will start seeing the boundary walls. But you will have to keep driving, after you enter the main gate, and cross the small village inside, till you reach the monument.
You will need your own transport to get to the fort. However I am sure that there is transport plying between Dina and where the fort is.
BTW: It was another General, Pervaiz Musharaf, who made this fort accessible for everyone by constructing a bridge over the river. Else one had to have a 4x4 or walk 2-3 km to reach the fort.