Sims 3 "Connecting" & Downloads Tab Error

goofygirl02​  If you do have access to a Windows PC with the game installed, remove or rename the Sims 3 folder in Documents > Electronic Arts if one exists.  This is to keep the content separate from anything else game-related on this computer.  Opening the launcher will create a new folder with no content.  You can trash this folder and restore the old one when you're done.

Download and install the Sims3Packs you want to use, a few at a time.  I'd suggest launching the game to make sure the cc works (and that the game itself works, since bad cc can stop it from loading).  When you're satisfied, you'll be transferring the .dbc files in DCCache, plus any files in Library (saved households and builds), saved sims (sims saved in CAS as templates), and InstalledWorlds.  You can zip these folders by right-clicking and selecting Send to > Compressed (zip) file.  Transfer them however you want: USB (formatted as FAT32 or exFAT) and cloud storage are the simple options.

Once the .zip files are on your Mac, double-click them to extract, then put the files, not the folders they're in, into the correct subfolders inside your main Sims 3 folder.  If you already have .dbc files in DCCache, you'll need to renumber the new ones so they keep counting up from what you already have.  For example, if your Mac has dcdb0 through dcdb4.dbc, the ones you transferred will become dcdb5, dcdb6, etc.  This has nothing to do with the .ebc files, which are Store content.  The Library and InstalledWorlds files don't need to be renamed unless you've already installed exactly that content, in which case you don't need to reinstall it in Windows.