how did you manage to turn it on?

Here's how you unlock it: http://www.mazda3forums.com/index.php?topic=76459.0 Mazda fixed this on the '08 though and the computer wasn't available on earlier model years.

Actually, all 2006 models include the fuel computer but the feature is "hidden" on all but the Grand Touring models. It can be 'unlocked' very easily using the instructions referenced in the previous posting. The fuel data is displayed in the same location as the clock once activated. Pressing the 'clock' button toggles between the clock and fuel info.

Thanks for tkanig the time to post. It's lifted the level of debate