CAUSE
This issue occurs when one of the following conditions is true:
• You created the Office for Mac document with the Microsoft Office for Mac Test Drive, and the Office Test Drive is in the Trash.
• You have more than one copy of the same version of Microsoft Office for Mac installed on your computer.
• The Mac OS X Finder could not locate the Office for Mac program that owns the Office for Mac document.
• The Office for Mac document has the wrong file name extension associated with it.
and solution is here
http://support.microsoft.com/?kbid=890889