Macah Encarnacao of Soliant Computing wrote a great little demo file for building folders on either a Mac or PC:
It is very common to generate reoccurring reports in FileMaker that get saved to the user’s computer. Perhaps you would like to organize these reports by day or type. OS level folders and subfolders are a great way to do this. This demo file attached shows you how to create a folder and a subfolder on your user’s computer using FileMaker.
The code has error checking, is easily modifiable, and is modular. This one's easy–get over there and download it for your tool chest. Thanks, Macah!