Ok, so I managed to run JDownloader 2 on my N900
It runs pretty well, but "Add new links" button isn't working so you can't download anything with it (LOL)
I hope someone finds a solution for this...

1- Install mmjre and icedtea6
2- Download JDownloader jar file from (http://jdownloader.org/jdownloader2)
3- Run it with your PC. Let it download all the files. (You will need JRE for your PC)
4- Rename downloaded <JDownloader> folder to ".jd" (without "")
5- Move ".jd" folder to /home/user/ of your N900 (without "")
6- Download "jd.sh" file and move it to your /opt/ of your N900
7- Open XTerminal as root and type ==> chmod +x /opt/jd.sh
8- Open new XTerminal and type ==> sh /opt/jd.sh
(WARNING: be aware, you don't need root for this command. otherwise you will brick your phone)
9- Please wait (a lot)
10- Tada!!!!

Update:
you can it run with ==> java -jar/home/user/.jd/JDownloader.jar
Add links button is working now!
it was mainly because of -Xmx512m which you can remove it from jd.sh script.

mmjre (https://garage.maemo.org/frs/?group_...elease_id=3649)
icedtea6 (http://maemo.org/packages/view/icedtea6/)

You can also make shortcut for it
Put jdownloader.desktop in /usr/share/applications/hildon/
Put jdownloader.png in /home/opt/usr/share/icons/hicolor/64x64/apps/