Hi Paulette--
You would use this the same way you use other callbacks. Write your _notify() function with a signature to match the window notify callback:
function _notify(win, dlgitem, event, data, appdata)
Make sure it's loaded at start up so the function is available, and attach it to the dialog window when you create the dialog, using window_add_callback(). See the online help for window_add_callback() for details on that.
Again, looking through the stock Arbortext code base (in $ARBORTEXT/packages/*) will be a good guide in how this is typically used.
--Clay