The first time I tried to install it on my DNS 325 it failed because my mac automatically renamed the downloaded fun-plug file to fun-plug.txt. This time when I tried it on my DNS 321 it succeeded. Down the files (and view instructions) from here for version 0.5:
http://ffp.inreto.de/ffp/0.5/
and instructions with example commands here:
http://www.consumedconsumer.org/2010/10/hacking-dns-323-and-dns-321-nas.html
Don’t do it! It worked for several days and then suddenly both hard drives are failed. I’m not 100% sure it is because of the hacking but it could be.