There isn't a 64bit build so you need to install the 32bit version using a compatibility wrapper called nspluginwrapper. I think the simplest way is to install flash from Debian Multimedia.
Code:
# nano -w /etc/apt/sources.list.d/debian-multimedia
deb http://www.debian-multimedia.org etch main
# aptitude update
# aptitude install debian-multimedia-keyring
# aptitude install flashplayer-mozilla
I, erm, have a little howto on my blog that includes a little more detail (plus setting up mplayer in iceweasel as well) although it was written for Lenny instead of Etch.
N.B. I apologise in advance if posting a link to one's own site is considered bad form here. I didn't see anything in the rules but I'm famous for missing the obvious.