To turn on the Developer Commentary you go to OPTIONS from the MAIN MENU. Then select GAME SETTINGS and go down to where it says DEVELOPER COMMENTARY, select with A. Then back out to the MAIN MENU.
Now select New Game, L.A. Meltdown and the Trigger is right in front of you, a little to the left. To turn on just stand next to it (Like you would a button) and press X. The achievement should pop after you hear them start to talk.