"Are you using NeoOffice or Office for Mac and have you found that you can't open a .docx formatted document?" asks John Carter. He has an answer for that. "One solution is to download Open XML Converter to get a .doc formatted file. Get it here. So what's the real difference between .docx and .doc? The .docx file is an XML formatted file and tends to be smaller than .doc.
"Open XML Converter can convert Word documents, Excel workbooks, and PowerPoint presentations that are in the Open XML Format so that you can open and edit the files in Office 2004 for Mac and Office v. X for Mac. (And NeoOffice or OpenOffice.)"