I am not connected with OGDW, so this answer is by no means the "official" word. But here is how I play it...
> 1) If the platoon starts game in the truck they are one or two units for activation table purposes
I treat the trucks and the infantry as two units. However, they must activate together while the infantry is riding in the trucks. (Same with infantry riding in halftracks, or on tanks, etc.).
>2) In the rules is written then when infantry in truck moves both truck and infantry has to perform actions.
> If I have one action per activation it means that in first activation I move trucks (and infantry inside) and
> declare that infantry will use their action later on. Then second activation will be spent to use it for previous
> infantry move inside the truck?
We have a platoon of infantry, and a platoon (unit) of trucks. If within command radius, both will receive an action and a bonus move during their activation.
Use the bonus move to move. Both the infantry, and the trucks, expend it this way. Then if you prefer, you can use the activation to dismount. Or if it is an open-topped vehicle they can spot, or fire, or communicate (if they have commo gear), etc. Or they can use their activation for a second move (so moving twice as far).
For the infantry to dismount (or to mount, in the opposite case), BOTH the truck AND the infantry stand need to expend their activation for the dismounting (or mounting) action. Infantry can't dismount from a truck that is not spending the time (and in the proper attitude) to dismount them. We don't have squads of infantry tumbling out of trucks as they role across the battlefield with their tail-gates up and locked.
If the truck did not move in that turn, then the dismount action can be followed by a bonus move. The infantry can move away from the truck, and the truck can withdraw (moving away from the infantry).
Once they have dismounted, in the next turn they will activate separately.
(aka: Mk 1)