The battery compartment cover is located on the top of the mouse. The seam running horizontally across the mouse is the top of the cover.

There is a button to pop off the cover. It is located on the bottom of the mouse. you may need to press hard on this button to pop the cover.

If the button does not work, you will need to pry off the cover. You can use your fingernails or a prying tool in order to do this, as seen in the third image.