3D Warehouse lives online, and you can access it via SketchUp or from a web browser.
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 going to Window > 3D Warehouse in the Window menu.
Following either of the above methods will open the 3D Warehouse browser window within SketchUp as shown below.
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.