If you need models for your SketchUp project, you can access the official SketchUp 3D Warehouse and get from there almost anything you need as this is the largest Sketchup models database.
To load a model from the gallery, download it first from the official website clicking on the button located on the lower-right side. It should look like this:
The file format of the file should always be SKP (the default format for the SketchUp application). If SketchUp is installed correctly, double click the file and it should open automatically in the application, then you can manipulate in all ways possible.