If the "patch" command is not working for you, it's likely that you may not have the "patch" utility installed on your server. The "patch" utility is part of a software package called "diffutils" (i think). It's presence is pretty common on most Unix/Linux systems, so if you don't have it on yours you probably should hurry up and get it.
First, make sure that you truly don't have the patch utility installed:
If you get something back like "/usr/bin/patch", then try patching qmail and use the full path to the patch utility:
/usr/bin/patch < /path/to/qmailrocks/patches/qmail-103.patch
If you don't get any results, then the patch utility might not be installed on your Debian box. The easiest way to solve this problem would be to install the patch utility via "apt-get". "patch" is part of the "diffutils" software package I believe, so you may need to do a little research to find out what the actual name of the package is to install. Alternatively, you can download the source tarballs and get more information at http://www.gnu.org/software/diffutils/diffutils.html
Hope this helps,