Sounds like you've got the not at all uncommon problem of not having installed BD correctly, David. What you need to do is first install BD 4 using the proper installer, and then unzip the latest BD 5 update on top of it, overwriting the files. For full instructions, and links to the download files, see:
https://wiki.mobileread.com/wiki/Book_Designer
Note that the icon will still say "Book Designer 4" even when it's BD5, but the "Help/About" menu item will then show you that it's v5.
RTF import should work just fine. Don't bother with DOC.