You installed ImportExportTolls NG, the add-on in Mozilla Thunderbird. In Thunderbird you go to Tools, but "ImportExportTools" or "ImportExportTools NG" is not visible. What do you do?
You need to have a mailbox -- even an empty mailbox -- configured. This will import all the messages you are trying to import into that mailbox. The hamburger icon's Tools -> Import, won't work. Once you have a mailbox configured, right click the top option and go to "ImportExportTools NG".
One way to configure a dummy mailbox is to use the trial version of Thunderbird Import Wizard here: https://www.emaildoctor.org/add/thunderbird-mbox.html
You can feed it (or utilize) a .mbox file to import 25 messages. This will get you started.