Take the music box out from the mat. Use a screw driver to open the thin oblong compartment on the top of the back of the box and install your batteries. Hope this helps. I had to call the company for help.
If you look at the element that lights up and resides between the two layers of fabric, you pull it out and on the sides of the yellow plastic there is a cylindrical part that houses the batteries. Simply twist that part and you will be able to take the batteries out and change them. Hope this helps!