Actionscript: address attached movies


I’m making a dynamic menu in flash, driven by a XML file.
I attach a movieclip using this code:

for (var i=0; i<items.length; i++) {
var item_mc = menu_mc.attachMovie("menu_item","item"+item_count, item_count);

I’d like to add a rollover effect using the mx.transitions class, but I don’t know how to address the attached movie?

I tried this one but it doesn’t work:

this.menu_mc["item"+item_count].onRollOver = function(){

Can anyone please help me out? Thank you very much!