3D Warehouse lives online, and you can access it from a web browser or via SketchUp 2017 or later.
In SketchUp, locate and click the 3D Warehouse icon in the top toolbar and a 3D Warehouse window appears that connects you to the main page, as shown in the following figure.
Alternatively, you can open the 3D Warehouse by selecting Window > 3D Warehouse from the menu bar.
When you open 3D Warehouse via SketchUp, you see a 3D Warehouse window, as shown in the following figure.
In your browser, go to http://3DWarehouse.SketchUp.com, and you see an almost identical 3D Warehouse main page, only, of course, in your browser window, as shown in the following figure.