3D Warehouse lives online, and you can access it via SketchUp or from a web browser.
In SketchUp, select File > 3D Warehouse > Get Models or select Window > 3D Warehouse, and a 3D Warehouse window appears that connects you to the main page, as shown in the following figure.
Due to our reliance on modern browser technologies for displaying 3D Warehouse features, it's no longer possible to use the 3D Warehouse web interface within SketchUp 2016 or earlier on Windows. The only workarounds are to upgrade to a newer version of SketchUp, which now uses our own embedded browser, or to use the web interface in the browser of your choice.
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.