I remember you really well! Tableau released Extensions with Tableau 2018.2 (“After Tableau 10.5, we are changing the way we number new versions of our software. Going forward, new versions of Tableau will be named based on the year of the release and the number of the release in that year(…)“). Therefore you can’t use Extensions before this release. Simply go to this page https://www.tableau.com/support/releases and download the latest Tableau Desktop (v2018.2.2 by today). As long as you’re allowed to install software on your machine (admin rights), you’re good to go.