You have three main options…
will work in a browser. You can find models you like and download them as a 2017 format file.
The Components panel has a 3D Warehouse search box in it, if you use that and find a model you want, click on its thumbnail, and the model will be inserted as a component.
You can use the sign in options, and enter your Gmail address as a Trimble ID. If you don’t know your password, try signing in as a Trimble ID on sketchup.com, and whatever password works there will work in SketchUp as well. Once you are signed in the regular 3D Warehouse window should work.