ZFW is calculated by adding the empty weight of the airplane, the PAX, and the cargo, without adding in the fuel.
With the Captain Sim 757 for FSX and FS9, I enter the GW in pounds on the designated line, (I think it is LS1L) and the ZFW is calculated by the FMC.  

72996LB would be entered as 730.0 since you round it up.  
The best place to get your GW or to figure out you ZFW is to go into the flight simulator menu bar at the top by pushing the ALT key and choosing "Airplanes" then choosing "Fuel and Load".  You will see all of the needed info there and you can set your fuel also.