If you are trying to import questions for your quiz, please check this article. For exams, please read here.
Actually, the structure of an export-file is different from that of an import-file. That's the reason that you can't simply upload an export .csv to import the questions.
The structure of the .csv-file that you use to import questions to your exam should exactly match that of the example-file.
If you have any trouble saving your Excel file in .csv (comma delimited) please check this article.