Look at the main display (the one right in front of you) as you press it. Various things will change depending on what mode you have the display in. I have mine in 2 gauge style and the lower right changes through the numbers for regen, miles since reset, avg kWh/100 mi, etc. I leave it on the instantaneous energy usage since the other numbers are all display in the center.
Also, using i-Guide look for "Using the trip computer."