Re: how to install custom worlds on mac?
@emiibelll Are you talking about a third-party world, not one from the Store or Exchange but one hosted on another site? If so, download the world, unzip it if necessary (so the file ends in .Sims3Pack), and place it in Documents > Electronic Arts > The Sims 3 > Downloads. Then open the launcher's Downloads tab, check off the world, and install it.
The installer may be hidden behind the launcher itself, but you'll know it's open if you have a second plumbob icon in the Dock. Click that to bring the installer window to the front.
For Store and Exchange worlds, the files should already land in the Downloads folder, so all you need to do is tell the launcher to install them. If nothing happens when you try, close the launcher and open it again.