C:\Documents and Settings\(User Name)\Local Settings\Application Data\Myst V End of Ages
Game settings are kept in the 'init' folder, savegames are in the 'sav' folder, and game progress is kept in the 'progress.db' file.
***Note: Local settings and Application Data may be hidden folders. You may need to change your view options to allow you to see and access these folders.
For Mac, savegames are located in the following folder:
Apple HD -> Users -> (Main User) -> Library -> Application Support -> Myst V End of Ages- > sav
To backup the savegames, make a copy of the entire 'sav' folder.