Once annoyance with the MUI Xtra is you cannot set a callback target - you have to create a global movie script function. Here is a script for extending the MUI Xtra. The purpose of this script is to allow you to specify a callback target for the MUI Xtra (it is a bit of a frankenstein hack - but its does its business fairly discreetly so you can create multiple MUI xtras without having to use global variables or global functions).