Try creating a Query and then for the field in question, in the criteria cell just place Is Null That being with a Space between Is and Null. Run that Query and it will give you results of all records without anything in that field. OR do it in SQL like this:
SELECT tblDateGrabber.PurchaseDate, tblDateGrabber.ToolNumber, tblDateGrabber.ToolName
WHERE (((tblDateGrabber.ToolName) Is Null));
Gives you the same results as using the above in QBE DesignView.
Hope this helps you. If not, paste a sample of your database and I will fix it for you.