The thread first post is in english and explains what to do
https://board.jdownloader.org/showthread.php?t=71069
1.) Download the betatest package
Windows (32/64Bit)
mega.nz/#!bUNzgA7L!8UQ4f0A33agG4LzUnqMy9RTT9S5JjP4p93DnF7oJv1c
2.) Extract the zip into your JDownloader/libs folder