I got it!!! There were two things I was doing wrong: I didn't know to include the output file name and I didn't realize that the file name (which was a long string of numbers and letters) included two underscores instead of just one. Once I corrected those two things, the program worked just fine!
Thanks for all your help--and thanks for not losing patience with me! I owe you guys one!
And now that I've converted my Mobipocket book into an lrf file, I'll be off to do a little of this ====>