You are creating the spelling list in Word 2007. Not everybody has Word 2007 and therefore they cannot open the document.
The answer is relatively simple. When you create the document click "Save As" and then select "Word 97-2003 Document" Everybody should be able to open that document type.
Jackie is right! Please re-save the "Spelling Long.....sounds.docx" file as a Word97-2003 document (it will then end with .doc), and re-post the homework.
Otherwise, those with less swept-up software will be unable to read (or get their children to do) the homework!