-( Story related, cannot be missed )-
After the ordeal directly with Joseph Seed, you'll find yourself on Dutch's small island. He'll give you some small objectives to perform to reclaim the island for the resistance, such as clearing out an encampment and rescuing hostages before you turn on the antenna and bring the island back to civilized society. Once you flip the switch on the antenna, you should gain this trophy and access to the multiplayer.