how to use wheel on Logitech mouse?

[Jinsong Zhao <zhaojs@Cadence.COM>]

I bought a set of the Logitech cordless Desktop. It contains a
wireless keyboard and a wireless mouse. The mouse has two buttons and
a wheel in the middle. According to the user manual, the wheel may
serve as the third button (in Windows), although the main purpose is
for Windows Scroll.

I set the ChordMiddle in XF86Config. Then run xev to see the
effects. Push the wheel yields no event. 

The only option for me now is to use the emulate. But it is awkard, if
you know what I mean.

Is there any setup for X to recognize the wheel as the third button?
Thanks. 

Jinsong