Using ditaval files with XML Author

I've been evaluating XML Author and am trying to determine how to use ditaval files with it. If I want to save a ditamap as a book for PDF creation, the Prompt for DitaVal File check box is greyed out. Does anyone know why this might be the case? Is the use of ditavals limited to a full implementation of FrameMaker?

I tried adding PromptForDitaval=1 to the [General] section of the ditafm.ini file to no avail.