You will need a variable, say current, to store which section is currently highlighted. It's essential, as when another section is selected, you must first restore the normal state for the previously current section.
See that thread (where you'll find an example to download) : http://forums.adobe.com/message/5410456#5410456
So, before playing the main timeline, your click event handler will execute something like that
sym.getSymbol(current).playReverse(500); // here, you restore normal opacity
current = e.target.getAttribute("ID").split("_");
sym.getSymbol(current).play(0); // here, you set reduced opacity