How are you connecting to the data?

If you don't have an ODBC connection and are using the Btrieve DDFs you will want to select FILE.DDF, FIELD.DDF or INDEX.DDF in the DBAMFG folder. Maybe someone can tell you the difference - they all seem to work the same to me.
 
If you are using a later version of CR, when you open "Create New Connection", select "Btrieve" rather than "Database files" as it will filter out any invalid files.
 
If you don't have them set up, I would strongly suggest creating ODBC DDFs and an ODBC connection for a couple reasons. It's generally quite a lot faster, so your reports take seconds instead of minutes. And you limit the available tables to those in the company you are using which makes it quite a lot less confusing to find the ones you want. There are detailed instructions elsewhere on this site (maybe on the Wiki too?) but basically there are only three steps - create the ODBC compliant DDFs in DBA, create a System DSN on the server, and finally a User DSN on any workstation that needs access.