1995 mitsubishi diamante... A/T mode?
 

there is a button that says A/T mode right below my shifter...

when i hit it a light that says "PWR" turns on

also, i have a O/D light on my dash but cant find the Overdrive button, where is it?

the power/economy mode merely changes the shift points at which it up and downshifts.
id recommend "power", as the car is much livelier, and the economy drop is negligible.
the overdrive button is on the shifter. dunno how you can miss it.

od button
 

well on my shifter there is the main button to changed from "P", "N", "D", etc...

under the main button there is a hole where it looks like another button could go but nothing is there???

about the A/T mode button it is a button and not a switch it is either pushed in or out...

when should i use the A/T Mode and when should i not?
what is the purpose of it? i drive 70 miles to work mainly freeway.... should i use it than or keep it off?

i also have an eco light on my dash but i am not sure what it has to do with and when it will come on?

just bought the car and do not know alot about it but would like to learn as much as possible!

The AT mode would be ok for towing a trailor or a load inside the car up hills where you would want the engine to shift at higher RPMs so it would not be shifting so much back and forth everytime you go up and down a hill, kinda like using OD off to keep a truck from getting in OD gear going up a hill with a load for better stable speed. It just makes the car shift at higher RPMs.