No option to load add ons to external drive

Ive only recently started playing sims 4 again and re downloaded all my old games onto my external drive for my Mac. The download went fine and the game runs well. The issue is that I bought the university expansion pack and went to manage my add ons and it was available for download but there were not options to change the location of the download to my external drive and it claimed that did not have the base game (because it isn't on my hard drive). All my other add ons downloaded automatically with the initial download.Help!