We have been through this before, I have watched your video completely. I realized whenever I open the spreadsheet I downloaded to add new records to it (ie, the the Festival records to attendee and stage spreadsheet) and when uploading to import there is an error message that says my file has been corrupted. Not sure what to do to avoid this.
I created a short video about using Google Drive (Docs) to modify the Stages.csv file so it can be imported to Salesforce. Follow the same process for the Attendees.csv file. This will let you finish this step without needing Microsoft Excel.