Urgh, got stuck at a bug. Too late to finish it tonight. So here you go with a few bulletpoints of the progress so far.
If you don't use HBCI you might just grab the current stable release since you're not affected by:
http://sourceforge.net/apps/mantisbt/jgnucashlib/view.php?id=18
Else download and extract jGnucashlib:
http://jgnucashlib.svn.sourceforge.net/viewvc/jgnucashlib.tar.gz?view=tar
Make buildme executable:
chmod +x buildme
Change the java path to fit your system. For me it was:
export JAVA_HOME=/usr/lib/jvm/java-6-sun/
Run buildme (takes a few minutes):
./buildme
Open the right file when jGnucashLibGui starts (not in .gnucash/books/ but wherever you actually saved your file)
Be annoyed and give up in the end anyway because of:
http://sourceforge.net/apps/mantisbt/jgnucashlib/view.php?id=19
Good night!