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I had this same problem at first, or at least I thought I did, until I realized I was using Tableau 2018.1. Once I opened files with 2018.2 (the release associated with extensions) I saw the objects. My confusion was complicated by the fact that I had both 2018.1 and 2018.2 on my desktop. So I thought I was using 2018.2, but my workbooks were launching with 2018.1.
I second this, make sure that you are using Tableau 2018.2 or later. You can find out your version by going to Help>About Tableau
Hope this helps!