I extracted the file again, filled-out the customization box, and it worked this time. Who knows? Thanks for your prompt response, Valloric. The reader looks great, and my books are definitely easier to read with Caecilia 55 Roman.
I didn't go with the Vera Serif since by looking at your graphic I wasn't sure it was that much of an improvement over Dutch. I had also considered Georgia, but frankly I didn't want to have to go through the process twice if wasn't satisfied with whatever font I chose. I've seen the Kindle and liked the font so I just made the decision to go with Caecilia. It's a huge improvement over the original Dutch. I'm quite sure Vera Serif would have been, too.
I appreciate your help. Thank you