Liquid Flash layout problem

I used the liquid Flash layout file from this site , but i got one problem.
I placed the script in a layer called actionscript and on frame 1, under it there is a other layer called background. I placed in just a square with a gradient color and made it a movieclip. I also placed this on frame 1. Everything is working perfect from this point but when i want to place a keyframe on frame 10 in the background layer and make a motion tween so that the background fades in from 0 opacity to 100, it doesn’t work.
It has something to do with the liquid Flash script. Does anybody know how to fix this ?