For PC macro installation & usage instructions, see:
http://www.gmayor.com/installing_macro.htm
On a side note, it's also possible the document has acquired some for of corruption or that your Office installation is itself faulty.
Corrupt documents can often be 'repaired' by inserting a new, empty, paragraph at the very end, copying everything except that new paragraph to a new document based on the same template (headers & footers may need to be copied separately), closing the old document and saving the new one over it.
You could also try repairing the Office installation (via Start > Windows Control Panel > Programs > Programs & Features > Microsoft Office (version) > Change).