Do you mean this Chrome addon or this Firefox addon from webextension.org/listing/download-with.html?
This addon can be used to send links to your JDownloader.
The "Download with JDownloader" addon is not developed by us thus we cannot provide any support for it!
Use it at your own risk!
Our official- and recommended 3rd party addons can be found here: https://my.jdownloader.org/apps/
