I'm not sure if the requirement type is not being mapped for you. I've contacted Mercury support about this issue and they have told me that there is a know issue with exporting from Excel, which has been escalated to their R&D dept. We have been able to export requirements, but there's nothing defined for the requirement type field in the excel add-in at this point. I wonder if that's what might be causing your problem?
What version of QC are you running?