I can't get the supplied example file to open in Flash MX either (I get "unexpected file format"), but I have got the effect to work. What you need to do is to make sure that your Movie Clip instance is named "bg_mc", as that value is hard-coded into the script:
Code:
this.onMouseMove = function() {
constrainedMove(bg_mc, 4, 1);
Attached is a screenshot to show you where you set the instance name.
Obviously, if your instance is called something else, you could alter the script to use that name instead.