The 3D Warehouse no longer supports SketchUp 2018 or earlier. In SketchUp 2017 you can try searching for your models in the Components panel and access them that way. If all you want to do is download them so you have them on your computer, you might find it easier and faster to go to your Warehouse account via your internet browser and download them from there.

Please update your profile. It shows you are still using SketchUp 8.

You have (probably) installed a 64bit version of Windows 8. Explorer can only use thumbnailers of the same architecture (64bit x86). You need an alternative thumbnailer, search the forum there are different solutions available (Mystic Thumbs, ruby scripts etc.).