Sorry I'm not too tech savy when it comes to mac but what do you mean by mount it? It is currently downloading as a dmg file
From my limited experience with Mac, when you double click a .dmg file it will mount it to your system automatically. If you open up the file explorer, you should be able to see it listed on the left hand side.
Maybe this could help you further: https://www.makeuseof.com/how-to-open-dmg-file-mac/