Hard to say without seeing the image... there's no telling when the forum queue will release the file to be downloaded.
My guess would be you need to select everything then use Pathfinder > Minus Front.
I don't seem to have a Minus Front - only a Minus Back. I've tried everything I can find on the Pathfinder tool bar but nothing seems to work. It obviously must but I must be doing something wrong. I've done a picture of the item. Square was created first, then circle, then star, but I want to know how to get the top edge of the star to go under the square like it does in the second picture (which I faked). Hope the pictures show ok.
The sample helps...
Here's a quick movie showing how. Pathfinder > Trim is actually best for that situation and you need to move the star backwards
It really all depends upon the art. Simply adding a couple points and removing the tip of that star point may be quicker. But for complex artwork Trim can be helpful.
Thanks! The movie shows it exactly (although I didn't think the point would have been deleted, but no matter). Cheers!