Sorry that I don't have more pictures for the single steps, because when I made this, I've never heard about instructables.com - but I hope you get the basic idea and it's a real simple project, so it shouldn't be a problem to make your own LED computermouse light.
And it's my first tutorial.
Step 1: The Parts You Need
an old computermouse
a LED (3 mm) with your favourite color - I used a white LED from the German store http://www.led1.de with 12.000 mcd
a resitor - depending on your LED - calculation at the next step
batteries - i used two 1,5 volt AA batteries
a battery clip - to hold my two AA batteries
Step 2: Calculate the Resistor
When you buy a LED you'll see UF for "U Forward" and IF for "I Forward" - these are the values you'll need.
So let's calculate the resistor:
R = U-Resistor / I-LED
R = 0,05 / 0,02
R = 2,5 Ohm
So therefore I used a 2,2 Ohm resistor.