Now that EW is officially released, I figured people are going to want to know how the new keywords work. Nothing is official till its in the FAQ, but they OK'ed me putting these out here as guidelines until a new FAQ is released.

Ambush X
The Ambush X keyword is active even when on a facedown development. During the Ambush step of the Battlefield Phase, the defending player chooses cards in the defending zone he wishes to Ambush and pays their Ambush cost (denoted by X), then they Ambush.
If the ambushed card is a unit, it must be declared as a defender for the current attack (it is not immediately declared as a defender when it ambushes). If the ambushed card is a tactic or has a “When this unit ambushes” effect, then it follows normal triggered action rules and must be put on the chain at the next available opportunity.

Raider X
Immediately after combat damage is applied, the controller of each surviving, attacking unit with the Raider keyword gains resources equal to its Raider value.
The Raider keyword triggers even when no damage has been dealt during combat, provided the unit with the keyword participated and survived.

Battlefield Phase

… Declare attackers, play actions, Ambush step, play actions, declare defenders, play actions, …

A couple errors/omissions in the Ambush rules, posted this so I could update the OP.

When paying ambush cost do you have to consider loyalty cost?

Nope, the cost to Ambush is just the Ambush number.

Does one need to pay the actual cost of the card when placing the card as a developpement, or when revealing it from face-down status?

Say a unit that costs 3 and two loyalty, with the Ambush 2 trait.

If i put it as a developpement, do i ever need to pay the cost of the card or will i only have to pay the Ambush cost?

Just the ambush cost. You never have to pay to play a card as a development.

If I am in the Ambush step and I've more cards that have ambush keyword played as developments in the attacked zone, I have to declare one by one these cards and pay its costs one by one or I must declare all togheter these cards and I pay the cost for these cards all togheter? In this step, my opponent may play a action like "long winter" on a development when I declare it for ambush?

I've been bugging Lukas about a better explanation of Ambush, hopefully it will be in the next FAQ. I believe you ambush with all ambush cards you want to at the same time, just like you declare all attackers and all defenders at the same time, but it's not perfectly clear from the current FAQ. The only thing you can do during the ambush step is trigger ambush abilities, you cannot take actions until the following action window. Which means the text of Test of Will should be slightly different, and that you for example can't ambush with Band Together and use the resources gained from its action to pay for other ambush costs.

Thank for your reply Mallumo.