These are the most current LAD rules:
http://www.sjgames.com/ogre/new-units/lad.html
And yep, it looks like you just say "I am activating it" at the beginning of your turn. It will spend that turn unpacking itself. You can fire it next turn. No die roll for activation - a palletized LAD isn't disabled and trying to recover. it's just folded up.
The rules text could probably make this a little clearer, but this is almost certainly how it works - basically steps 2 and 3 of the unloading and setup process.