I just tested it with the latest version from the app store and I don't get this error. Could you remove the module from your project and download the latest version of the Excel importer?
If that doesn't work, then create a new project and download the mx model reflection + Excel importer and see what happens.
Let me know what the results are
Try saving your excel sheets using this file extenstion, .xls, this may be a temporary workaround. I think there are some compatibility issues with the new Excel Importer and the .xlsx extension, which is the default XML-based file format for Excel 2007-2013. We were experiencing the same issue when trying to import Excel documents, but when we changed them to a different file format we had no issues. Hope this helps.