12-25-2009 02:04 PM
I had the same problem. After a few hours of playing with it, my DH finally found out how to pop it off.
The battery door is the piece that has the Logitech logo on it. Hold the mouse upside down with the battery door release button at the top. Make sure you are not covering the battery door with your hand. Depress the button until the entire button goes in, especially the (now) top part. You may need to use your fingernail to pry it off the first time. Pry under the door where it meets the bottom part of the mouse, not the part where the mouse buttons are.
I've now opened the door several times, and once you've done it it gets easier.
Good luck and Merry Christmas.