In our OnDemand Ariba solution (v10) in the Refine Data screen, there is a hyperlinked titled "Customize this form." This wil take you to a new screen where you can add the fields that you would like to filter on. Click the Done button after you selected the fields you would like to filter on. You'll be returned back Refine Data screen where you'll be able to set your fiters for the fields you selected.
Hope that helps!
Ed, Check out Chapter 6 of the "Ariba Sourcing and Supplier Management Reporting and Analysis Guide" and it details the different ways to filter the data. David Morel