I have had this before on osx. If i remember rightly, you have to close MoI then open it as if starting a new model, then use the file/open to browse to the file and then open it from there. After doing things this way it exports again. I don't know why it does it, might be a file permissions thing. Just remember not to double click step files even if they are correctly associated - It could also be a folder structure that's not carrying over properly via wine. It's a nuisance though.

Martin Spencer-Ford