Imported COLLADA models don't appear on Mac.

I have been using Google Sketchup to create 3D models for my Unity project, and have been working with Windows and Mac. When imported as COLLADA files, there are no problems on Windows, but on Mac, the models simply don't appear. The files can be opened in Preview just fine, but nothing shows in Unity.

Can anyone please provide any insight, advice, or instructions on why this happens, and how to fix this? If specific information is required, let me know, and I'll try to provide it.