If your installed Expansion and Stuff Packs aren’t showing as installed in your Applications > Electronic Arts > The Sims 3 folder and you are receiving the Unknown Error message then you need to follow the instructions below to get your packs working properly. This is related to, but not the same as, the other Unknown Error message whereby files need copying from one EP to the other. You will also need to probably follow those instructions in order to complete the installation successfully. Get yourself comfortable before doing this, depending on how many EPs and SPs you have it may take a while!
Symptom:You have just downloaded and installed an Expansion or Stuff pack through Origin but the pack is not showing up in your Applications > Electronic Arts > The Sims 3 folder. If you try to launch the game from another EP/SP or the base game you get the Unknown Error message.
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Affected Expansion and Stuff Packs:Ambitions, Fast Lane, Late Night, Outdoor Living
Affected Macs:All those downloading and installing the digital versions of the affected packs through Origin.
Cause:During the installation process the add on pack alias (shortcut) is not being added to the base game Application folder so the game cannot find it.
Navigate to your base game in Applications > Electronic Arts > The Sims 3, right click on it and then select Show Package Contents.In the Finder window that opens navigate to Contents > Resources > AddOns. Here you will see the icon for the packDouble click on the pack icon you are missing. You may get a popup telling you an update is available, go ahead and update. If you are further prompted to update Permissions then go ahead and enter your password when prompted (this is a macOS security feature).After the update you will probably get the Unknown Error message again. Just click ok.Now right click on the add-on pack and select Make Alias. Drag the alias file you have just created into the Applications > Electronic Arts > The Sims 3 folder so the folder now has the base game and all your EP/SP shortcuts (aliases) in it. Delete the alias from the file name so it just has the name of the EP/SP.Repeat the above for every EP/SP that has not installed into the Sims 3 Applications folder.