To access the DLC you’ll need to find the pack on the PlayStation Store and manually download it. Once done, you can then find the DLC mission via the list of quests in your Pip-Boy. It's probably best if you are a higher level when playing, so bare that in mind.
To get started listen for the distress signal via your Pip-Boy Radio. Listen to the message once and this should update your quest log. You'll then have to “locate the ambush” and follow the rest of the instructions to begin the mission.