If you own the Botanica content pack for Anno 1800 either through the Anno 1800 Season Pass or by buying it separately, you will be able to access the new content in game.
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 at least 1000 engineers. Once your population surpasses 1000 engineers, the buildings will unlock in the Building menu.
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!