Weird, my guess it has something to do with your browser or your computer cause I can extract it without a problem.
I'm downloading with FireFox, on a Windows XP Pro SP2 Dutch and I can open it with either Winrar or Powerarchiver 2004 correctly.
One last try, I packed the files on 4 different ways (and I repackaged the zip file with Winrar) so I suggest trying one of these:
http://uploads.code-area51.com/Pack-Fission.zip (zipp'ed version)
http://uploads.code-area51.com/Pack-Fission.tar.gz (tar'ed version)
http://uploads.code-area51.com/Pack-Fission.exe (self-extracting archive)
http://uploads.code-area51.com/Pack-Fission.rar (rar'ed archive)
If these don't work, then I can't help you!