If you own the Docklands content pack for Anno 1800 either through the Anno 1800 Season 3 Pass or by buying it separately, you will be able to access the new content in game.
Docklands in Building menu
Docklands info in main menu
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.
Import prompt
In order to access the new content, your current city requires to have at least 250 artisans. Once you reach enough inhabitants for citizen tier III, you will be able to access the Docklands buildings from the Building menu.
Docklands in Building menu
Should you be unable to access the new content, please make sure that the game is fully updated.
You can find more information to Docklands on the dedicated Anno Union post.
Feel free to get in touch if you have any further questions!