The files available for download from this page are covered by The Udanax Open-Source License, which you may want to read before proceeding.
The Udanax Green backend, written in C, and frontend, written in Python, can be downloaded from udanax-1999-09-29.tar.gz (a gzipped tar file) .
% tar xzf udanax-1999-09-29.tar.gzUnder MSWindows, the shareware WinZip program does a fine job on .tar.gz files. Alternatively, you can download the excellent Cygwin environment, which gives you a Unix-like bash shell and commands, so that you can issue the above tar command. If you're going to proceed to develop under MSWindows, we strongly recommend Cygwin.
After downloading and unpacking, do take a look at the README file.
You will then want to download and unpack the 2005/2006 updates: udanax-patch.tar.gz
(For historical interest, udanax-oldfen.tar.gz is an old frontend written in C for the NeXT platform.)