Photoshop elements serial number issue

You should contact your sales person (so: Staples), not necessarily Adobe. If the sleeve says that a serial number should be included but there isn't any then it almost seems like they sold you an 'empty' box. Not really empty but basically they sold you the media but not the license. Unfortunately this sort of thing happens more often, I've seen the same stuff happening with audio equipment (which should have been sold with specific software) or with audio software (which isn't sold with a serial, while that should have been the case).

Of course I can't tell if this is also happening at Staples, it sounds kind of bizarre to me, but for now you should definitely get in touch with them first because they sold you the product.