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 story mission and region your current city requires at least 700 artisans. Once your population surpasses 700 artisans, you will get an expedition mission - similar to the one you got for unlocking the New World. As soon as you have completed the expedition successfully you will be able to access the new region - Cape Trelawney.
Should you be unable to access the new content, please make sure that the game is fully updated.
Feel free to get in touch, if you have any further questions!