This happened to me last week. To import you must first export. To do this you must first create something to export. Do this by modifying one of the models - any one; this apparently creates something that can be exported. Cant remember exactly how the export went but it was simple - and it did not "damage" the model file at all - I think the modified model could be reverted or deleted.

I'm away from my computer and notes, so this sounds a little jerky, but once you export anything you will be able to import - at least thats what happened to me.

If this sounds strange, and there are no directions about this anywhere in RF (I found this info in the Knife Edge forums), it blew my mind somewhat.

It did however solve the problem - hope it does for you as ther is quite a lot of models and sceneries in the swap pages at KE.

Good luck