Here's a discussion thread from mazda3forums about it.

The procedure is:
-With car off and doors closed insert key into the igntion but do not turn it.
-Hold down the CD and AM/FM buttons
-Turn the car on
-Wait about five seconds or so and the LCD will display "DIS ON"

The owners manual has info on how to use the DIS stuff, though your HU will have a ......... SET(?) button instead of an "INFO" one.. they do the same thing though. It will let you see the average MPG, current MPG, and miles to empty.

 

Put the Key in the ignition, hold down the AM/AF and the CD button at the same time, start the car holding the buttons down, you should see DIS ON in the radio display, turn off the car and then restart it, hit the set button and you will see Avg MPG, Current MPG, Miles to empty and average speed. Hit the clock button and you have the clock, set button and you are back to the trip computer.

 

Just did this on my 08 M3i, and it works!

Two things: Depending on the state of your car's parameter RAM, you might have to disconnect the (-) terminal of the battery for a couple of minutes, then reconnect it, before trying to enable the trip computer [note - this will reset a lot of your info like radio presets and the power window one-touch]. Also, turning the key to ACC will allow you to turn on the display (DIS ON) but won't allow you to switch to the trip computer reasouts (at least on mine). You'll need to turn the key fully on or start the car before it'll let you switch to the computer display.

 

Cool...