Patcher is a standard XEmacs package. As such, you can download and install it directly from a running XEmacs session. The packages interface is accessible from the ‘Tools’ menu or via M-x list-packages). You may also manually download a tarball or use the Mercurial repository. See http://www.xemacs.org/Develop/packages.html for more information.
Otherwise, Patcher is also distributed as a standalone package directly from my website (a Git repository and tarballs are available), at http://www.lrde.epita.fr/~didier/software/elisp/misc.php. You will also find different inlined versions of this documentation at that place. For standalone installation instructions, please read the INSTALL file in the distribution.