Instructions for downloading the XML File Format Converter for Mac, which allows Office 2004 to view and edit documents created in Office 2008/2010.
In response to complaints about backward compatibility, Microsoft has created an Open XML File Format Converter that allows Office 2004 programs to view and edit documents created in Office 2008/2010. As more people upgrade to the newest versions, they may still want to share their work with individuals using older version of Office. The XML Converter allows for seamless sending and receiving of information.
Installation and download instructions can be found on Microsoft's Support KB.
Information about compatibility for Office on Windows can be found at Microsoft Office - Viewing Office 2007 Files Using Older Versions of Office.