As mentioned in the preface, editor runs in your browser. There's no need for any special plug-in, just JavaScript support enabled. Everything is accomplished using dynamic scripting. Due browser diversity, it's very difficult to maintain full functionality for all popular browsers.

In fact, there are currently only two browsers fully capable of running Amy at 100%. Firefox and Safari - arguably two best browser implementations so far. Since Firefox (or any Gecko based browser - Camino, Seamonkey, …) is available on any platform (and usually loved by web developers) this shouldn't be such an issue. As for Macs, it works perfectly in Safari, the Apple browser flagship (Safari is in fact the best suitable browser at the moment). Windows users with Internet Explorer are somehow limited to the latest 7.0 version of their browser, which has some unfortunate quirks in event handling preventing 100% support.

Speaking of Opera - it would be perfectly capable of running Amy, if only its zillions of key shortcuts were disabled (at least the single-key ones).