I was asked recently if you can access Path Variables from within InstallScript. The answer is no.
Here is an except from the InstallShield Help:
"Path variables are used during the development of your installation project. These paths do not apply to the target machines where the application is being installed. Rather, they are used to link to source files for your installation project. When the project is built, those links are evaluated and the files they point to will be built into the installation."