How to know Installation Folder?

In the Actionscript 3 code of my AIR application, I’m writing dynamically to an xml file which will be created in the installation directory of the user’s system (\Program Files in windows AND /Applications IN MacOS) at the time of installation. When I’m running my app, I’ll be writing to and reading from this file.
My problem is that I’m not being able to find the path of the installation directory in my code so that I can use that to access this file.
Please help…