If your just visiting for a few months just carry your american license and Paki ID card, ie NICOP or whatever you have and if you get stopped just explain your visiting etc this is your license from states, but make sure when you go back to usa to sign up for an international driving license.
If your going to stay in Islamabad for long term than it would be a good idea to go ahead and get an Islamabad driving license though because some motorway police say that american drivers licenses are not valid in pakistan although I doubt they'll seriously do anything to you.
Also if you get a traffic violation in pakistan, they usually take your car documents or drivers license as collateral until you pay the fine in the bank, than you have to go back to the police station, show your receipt to get your license back as well. Best of luck