I've got a newer micro wireless and it is a little tricky, but here's how to do it:

First, the computer has two buttons: one button is located on the center bottom of the front of the computer and the other button is essentially just pushing the entire computer down and it makes a nice "click". If you push the computer down (without pushing the other button) and hold it for a couple seconds a screen comes up that says:

ON
Slct

with a little picture of a sun or something in the upper right corner.

So just press the face of the computer down and hold it for another couple of seconds and the screen should go back to normal. Now, if you press the face down the backlight will come on and stay on for a little bit.

When you want it to go off, just press the face down again until you come to the same screen you saw before. Make sure the selection says "Off Slct" and hold the face down once more until you go back to the main screen. Or you can just let the computer power down after your ride is over and the light will automatically be turned off.