Adapted from Linux Forums
The flash plugin is available from 2 different packages: one called "flashplugin-nonfree" from the "non-free" section of the official Debian repository, or the other commonly known is the one by Christian Marillat, which is called "flashplayer-mozilla".
For details on how to add the Marillat repository (in your /etc/apt/sources.list file) : debian.video.free.fr
As for making it work, ... Konqueror will scan the Mozilla plugins. If not, just go in Konqueror options and do a manual plugins scan ("plugins" section of the options).
Once you have added the Marillat's repository to your /etc/apt/sources.list file, you simply type:
apt-get install flashplayer-mozilla
For example, if you have Debian Testing, your etc/apt/sources.list would include the lines:
deb http://www.debian-multimedia.org sarge main
# deb-src http://www.debian-multimedia.org testing main