I'm posting this comment here because this thread is the most wonderful and comprensive set of troubleshooting instructions I could have asked for. And I'm a lifeling mac user, so this is not the first (or tenth) deep dive into the forums. Thank you to BlueBellFlora, Tremayne4260, and all of you who put in so much work to make my gaming experience possible again.
There is one small problem that I never saw addressed, although the solution is probably obvious to people who are more computer literate than I. When I bought a new macbook pro a couple of days ago, seemingly every step of installing the Sims caused an issue. Most were solved by BBF's many troubleshooting guides, but I was still unable to download any content from the Sims store. Whenever I pressed the download button, I got the window screengrabbed in her "downloading store content" guide and intended to follow the instructions not to touch anything. Firefox opened another tab, as described, but another window asked me to choose an app to launch. Selecting the default "The Sims 3," with no icon to its left, gave me the "unknown error occured" message whether the launcher was open or not. (I never had any actual game running.) Waiting for ten minutes did nothing. Manually choosing the sims3 app with plumbob icon from my applications folder gave me the same error. However, selecting the sims3Ambitions icon (the one which launches my game) worked fine, and everything proceeded smoothly from there.
I know that this is probably simple, but I wanted to point it out for anyone who is stuck like I was. I wish EA actually cared about fixing its bugs. but that would cost money that they then couldn't pay themselves. Of course, BlueBellFlora is probably smarter than their entire debugging team put together. Thanks again.
