The 2 buttons on the right of the clocks change the brightness of the illumination - it's the 2 on the left you wanna be pressing.
The top one should scroll through the following (it does on UK cars).
Current Speed
Outside Temperature
Distance To Empty (dte)
Average Fuel Consumption
Elapsed Time / Stopwatch
Up Shift Indicator (Manual only). This can be changed by pressing the lower of the 2 buttons.
If you haven't got a user manual, just Google 350Z Owners Manual PDF. The info you're after is on p.68 (in the one I'm looking at anyway) Lthumbs:
Maybe that's why i cant find it my car is automatic.