Do you by any chance have something specified in the HTMLTidy path option? Try removing that. It should be set to the full path of the directory containing tidyhtml.exe, or blank. Anything else will cause this error (I need to make this more robust, I admit). You may have an installation where the .exe is named "tidy.exe"; in this case rename it or copy it to "tidyhtml.exe". I will change the code so the next version lets you specify the path of the executable rather than the directory.
The Librie DLLs are bundled in the installer. Be warned though that they seem to be very fragile; in many cases they just blow up when you feed them HTML. They work for simple stuff only. In the future I'll probably run the HTML through the internal PDF/RTF converter but just to simplify the HTML first, so it contains just basic tags. That should help.