I didn't test any of this path stuff yet, but for Windows, you can retrieve the user's MyDocuments using the API:
WCHAR path[MAX_PATH];
HRESULT hr = SHGetFolderPath(NULL, CSIDL_PERSONAL, NULL,
SHGFP_TYPE_CURRENT, path);
But, I don't think it is the right approach. Normally, mobile devices works with the sandbox concept, which the application can have access to its own space + specific shared spaces, like photos, etc...
At first, AppGameKit should have a "AppStoragePath", "ApplicationPath", "SharedPath" (I made up the names), and any AppGameKit application should only use those and, if needed, add relative paths to them.
For example, flash & actionscript uses the following:
File.applicationStorageDirectory—a storage directory unique to each installed AIR application
File.applicationDirectory—the read-only directory where the application is installed (along with any installed assets)
File.desktopDirectory—the user's desktop directory
File.documentsDirectory—the user's documents directory
File.userDirectory—the user directory
Of course, some of them are not applicable for all platforms. In any case, we shouldn't use absolute paths, ever.
Cheers
Paulo