Adventure Creator is an asset for Unity that is available to purchase on the Unity Asset Store. Once added to your account, you can download it from the Asset Store. Within Unity, go to Window → Asset Store to open up the Asset Store window, and then click the Assets icon in the top-right. Search for "Adventure Creator" and import it.
In Unity 2020.0 or later, you can instead install purchased assets from the Package Manager. Choose Window → Package Manager from the top toolbar, and opt to view My Assets. From there you can search for "Adventure Creator" and import it.
The full package includes two demo games, placed in the AdventureCreator folder's Demo and 2DDemo subdirectories. While its recommended to include them so that you can refer to them when building your own game, they aren't necessary for AC to install, and you can omit them from the import process if you wish.
One imported, a box will popup informing you that AC needs to define an Input named Menu, and three Layers named NavMesh, BackgroundImage, and DistantHotspot.
Click OK to let AC make these changes, and installation will be complete. If you opted to import the demo games, you can run them via the top toolbar. Otherwise, you can use the New Game Wizard to start creating your new game.