Ideally this conversion should be done by the function built into the DLL and you should directly receive the port address as a 16 bit signed integer. But again, since I am not aware of the internal working of the functions used in the DLL file, I can not be sure.
Currently i do not have LV 7.1 installed. I used LV 2010 and got the following error while loading the DLL:

If you are certain this is a LabVIEW issue and not dll's you should contact NI support.
I will try to load this on an XP system with LV7.1 and let you know the observations.