ATS appeared on the scene somewhere around 3.3/3.4.
It seems to be included in the 3.4.10 download on arsdigita.com. Unfortunately, I don't have a stock 3.4 server handy to test it on, but I know I used to use back in 3.x days (it was still under development back then so the way I was running it then isn't the way it should be run now). Take a look at /doc/templates/ on your 3.4 server and read up on templating, then have fun and ask again if you can't get it working. (The code is in /packages/templates, *not* in /templates/, which is an older temlpating system for ecommerce that is not nearly as cool as ATS)
