Item level auditing doesnt exist in SharePoint. I know that article by Ted Pattison -i read it several times before- but it just doesnt work on SPListItems. I have it first hand from CSS (MS Support) that it was chosen late in the dev process to not include item level auditing for performance reasons.
However since you want to enable auditing on a document library, you are in luck -as long as the library isnt a publishing page, since auditing is completely broken on Pages libraries. Read this other article on MSDN on how to enable auditing on a single list. It cal also be done using the GUI if you are using MOSS.
For more on this, check my answer on Stack Overflow on the same issue.