Model import problem!

I downloaded some models from tf3dm.com and when I imported them, half of them didn’t even work. I tried dragging them into the scene and they wouldn’t work, I think they were 3ds files or something like that. In the Project View, instead of showing the model, it showed an icon of a white page. Some of these models are some that I really need! How do I fix this?

Unity3d accepts this types of files:

If you have problems with .3ds .dae .dxf .fbx or .obj , I recommend use FBX converter from autodesk. Is a free tool and you can get here:

I don’t know what to do with .c4d

Un saludo!