When launching Age of Empires II: Definitive Edition, to access Return of Rome, you will need to select the Return of Rome icon on the Main Menu to transition your game.
You will be taken into Return of Rome, where you’ll be able to access favorite and new content, including the Age of Empires classic civilizations, brand new campaigns, and a new competitive D3 game mode.
Purchasing Return of Rome also grants you access to the bonus Romans civilization while playing in Age of Empires II: Definitive Edition mode.
For more information about what is included in the Return of Rome check out these blogs:
Having trouble downloading Return of Rome? Follow the steps in the Trouble downloading an Age II: DE DLC article.