Flash MX 2004.
library.newFolder([folderPath])
folderPath A string that specifies the name of the folder to be created. If it is specified as a path, and the path doesn’t exist, the path is created. This parameter is optional.
A Boolean value: true if folder is created successfully; false otherwise.
Method; creates a new folder with the specified name, or a default name ("untitled folder #") if no folderName parameter is provided, in the currently selected folder.
The following example creates two new library folders. The second folder is a subfolder of the first folder:
fl.getDocumentDOM().library.newFolder("first/second");