The latest release can be found on in the ljupdate download area.
If you'd instead like to use the CVS version, check out the ljupdate project page for instructions.
I recommend using the CVS version; development is fairly stabilized.
Keep up with the latest developments by reading the ljupdate LJ community, and/or by reading the ljupdate mailing lists.
If you're running an older Emacs, you might not have the MD5 library installed. You'll still be able to use ljupdate, but if you do, it will be very, very insecure. I'd recommend either upgrading to GNU Emacs 21 or installing md5.el, which is included under contrib/ in recent ljupdate tarballs.