The mdforward distribution can be obtained from http://netherstone.com/nemeton/sw/mdforward/mdforward-1.1.tar.Z
The easiest way is probably to ftp this to the server you intend to install it on, and unpack it there.
mdforward has only been tested on Unix systems under sendmail. While there is some support for qmail other mail transport agents and operating sytems are quite untried waters. Reports welcome, though. :-)