Assets aren't just 3D models; they are also materials. D5's native material library requires online access. To bypass this for free:

Once you have imported custom models, you can save them to your own within D5: Select the imported object in your scene. Right-click and select Add to Local .

By default, these assets are or cached . You click a tree, D5 downloads it temporarily. If you clear your cache or switch computers, you have to download it again. This is designed to save hard drive space, but it is a nightmare for offline workflows.