I think there are a few different versions of quizlet import, I was able to use this one successfully to import a private deck with the instructions on the addon page.
https://ankiweb.net/shared/info/1362209126
For private decks, you first have to set up with with cookies or qlts in the add-on config. You get the info from logging into Quizlet, going to the developer console of the browser. In Windows, this is (from Chrome and most browsers) F12 or Ctrl-Shift-I, then click on the Application tab > Cookies > https://quizlet.com and grab the whole value for qlts at the bottom. Here is a screenshot, except I have blurred the actual value.
Copy this Cookie Value from the bottom and go to Tools > Add-ons > Quizlet to Anki 2.1 Importer with audio support > Config and paste it inside the double quotes for qlts.