DOS applications don't understand the longer filenames commonly used under Windows. So when saving your files, you'll need to restrict yourself to using only eight characters, plus a three-character extension, like this: CARSALES.WKS.
You don't have to worry about forgetting this requirement; the program won't let you use more characters in your filenames. But if you need to export a file from some other program and then use it in your DOS program, you'll need to remember the eight plus three restriction.