I found an answer to my question in the errata for the book at http://support.apress.com/errata/1861003145_errata.htm

Missing code file
This is regarding the missing demo.dbm file in the code download for this book. Since this particular sample file is not available in the code bundle, you could instead use some other sample DBM database file to run the code. You could also create a sample DBM database following the instructions on page 437 and use this file in the code.