When you load an existing save game, you will be prompted if you would like to import the new content. Confirm the import to access the new content in your current game session.
In order to access the new content, your current city requires needs to have at least advanced to citizen tier IV - engineers. Once you reach the tier IV, a newspaper article and the discovery expedition for The Passage will be unlocked. As soon as the expedition was successfully completed, you will have access to the new Arctic region and gameplay.
Should you be unable to access the new content, please make sure that the game is fully updated.
You can find more information to The Passage on the dedicated Anno Union post.
Feel free to get in touch if you have any further questions!