You didn't say exactly what error you were getting, but assuming it was because you set it up incorrectly you can just follow this post from the old Steam forums;
https://web.archive.org/web/20150219053513/http://forums.steampowered.com/forums/showthread.php?t=933418 It's only briefly mentioned in that thread as far as I can see but you do tend to need to set the duration in ALChemy to lower than the default of 25 (I think just 5 does the trick), otherwise you get glitchy audio. There is also a bug in the game that causes EAX to stop working between game launches and the only workaround as far as I know is to turn it off and then back on again in the settings every time you launch the game.
As far as I'm aware, the EAX implementation in The Witcher was always a bit buggy and CD Projekt opted to replace it rather than fix it in the end. There's an option called 'Environment Effects Enabled' directly under EAX that turns this replacement on and off. Needless to say compared to full hardware accelerated EAX, it sounds kind of crap. Just a shame the EAX stops working between game launches; I would love to know if there is a way to fix that permanently.
