Just letting you know that the 3DW website is currently experiencing some technical issues. In short, uploading functionality is in an unstable state and you may have to wait to upload new models. However, downloading and viewing features should keep working.
3D Warehouse is the online repository of SketchUp models. No need to “install” models into 2018. If you have downloaded them, you can just open this in 2018, just like you would have in 2017.