JComments is an extensive comment extension for Joomla. To use it with the jDownloads series 3.9 you must download this file. Afterwards copy the unzipped file, com_jdownloads.plugin.php, to: components/com_jcomments/plugins.
Please also see the jDownloads Documentation (opens in a new window/tab) for more details.
Make sure that the option "Activate support for all Joomla content plugins?" is activated in the jDownloads configuration Frontend tab
Comments are only shown in the Download Details
Do not use Komento and JComments at the same time!