With Office 2007 (Win) 2008 (Mac) and later, Microsoft has moved to new file formats for documents – .docx instead of .doc for Word, .xlsx for .xls in Excel, .pptx for .ppt in PowerPoint. Older versions of Office applications cannot open the new format files. Microsoft has released some updaters that give the ability to open the new format files in Office X Mac, Office 2000, Office 2002, Office 2003 and Office 2004 Mac
Win: Office Compatibility Pack – Instructions and download here
http://office.microsoft.com/en-us/word-help/open-a-word-2007-document-in-an-earlier-version-of-word-HA010044473.aspx
Mac: Open XML conversion tool
http://www.microsoft.com/mac/downloads?pid=Mactopia_AddTools&fid=0C323A12-6385-4666-AD39-A9516A8EDA14#viewer