The Amazon-supplied kindlegen utility is required in order to produce final-form Kindle books.

While an EPUB is just a Zip file, a Kindle book is in a proprietary format that can only be produced from the command line using the Amazon-provided kindlegen utility. The kindlegen utility is a free utility but is proprietary so it cannot be packaged with either the Open Toolkit or the DITA For Publishers plugins.

To get kindlegen, google "kindlegen" and the Amazon download page will probably be the top hit.

Download the package and follow the installation instructions (which should be to simply unpack it somewhere convenient).

You can either put the kindlegen executable's directory in your PATH environment variable or you can specify the full path to the executable using the kindlegen.executable Ant parameter (see kindlegen.executable Ant parameter).