Stuffit Expander
AccountEdge and AccountEdge Basic now exclusively use Zip archives, and no longer support Stuffit archives. You can unstuff the Stuffit .sit archives with another method. If a backup was created in an older version of software, you can use that (older) version to restore the file.
Alternatively, a free Stuffit expander is available from Stuffit. http://www.stuffit.com/mac-expander.html