If you are a Mac user with an older version of MS Office (e.g. 2004 or older) you may find that you are unable to open files created with MS Office 2008. This is especially true if you are trying to use the new batch download feature in ARTstor, which creates PPT 2008 files.
A converter plug-in from Microsoft should resolve these file conversion issues. Follow this link, and download the plug-in at the bottom of the page where it says 'Open XML File Format Converter for Mac 1.0.2'.