To open the mouse first remove the ball and cover. Then using a small screwdriver pry up the blue plastic trim. Remove both trim pieces and this will reveal two small philips head screws (remove). Now the top and bottom halves can be separated by pulling them apart. This takes a bit of work the first time. Now that you are inside the mouse, solder on a LED to the main board as shown. The voltage here is 5 volts. Radio Shack sells a Blue LED that is designed to run at 5 volts (part # 276-311) This is ideal because no dropping resistor is needed. When connecting the LED, connect the negative side to the pin #4 (right side) and the Positive side to pin #1 (left side).
Reassemble mouse and plug into computer, you now have a lighted mouse!
A fellow named Kevin wrote the submitter of this page with a slightly different method of adding a light to an iMac mouse. The above picture was taken using this method "I felt the intensity of the LED pointing upward was too concentrated, so I popped the mouse open again and drilled a small hole into the plastic housing where the ball resides... that way I could point the LED straight into the ball chamber. I get more of a diffused lighting effect and you can even see the ball rolling around."