Hi Neil,

Autosave works fine on my system.
users/*myname*/documents/ Autodesk/Autosave is not a file, but the path to the folder with name "Autosave".
If you don't have such folder on your Mac, try to create it, than AutoCAD will be able to use it as autosave folder.
Alternately you can create special folder inside your Home folder for AutoCAD autosave and change the default path for Autosave folder in AutoCAD preferences to this newly created folder.

Maxim