I am not familiar with Modview, so these may not apply, but the most common things I've found that will stop a mesh from being loaded into a viewer/editor are:
Too many polys. Some are limited to 17k polys.
No UVW coordinates or material applied.
Some obscure incompatibility that hasn't been addressed yet, such as different versions of physiquing tools used. This one is rare when compared to the first two though.
EDIT: I was way off base. I just found a thread which said it is due to the file path being too long and that the mesh should be moved so the path is shorter... maybe desktop temporarily?
Apparently it worked for the person who had the same issue.
