Washer/Dryer Prep doesn't include the vent through the outside wall as many (most?) people never install the washer/dryer. The installer has to install the vent. You'll also want an overflow tray and overflow drain hose installed, even though it's unlikely to ever have an overflow.
There may be a sticker on the inside wall showing where the vent goes. Even with that, you should take a very small drill bit and drill small holes from the inside (not all the way through) where you intend the vent hole to go to ensure you're not hitting a sidewall frame member. When drilling the hole from the outside, most people run the drill in reverse to score/cut the fiberglass exterior surface before cutting through the wall.