@Yzmaevile Make sure that all the custom content is in .package format, not .zip, .rar, or .7z. These other formats are various types of compressed files, and the game can't read them. Both Windows and macOS can open .zip files on their own, but for .rar or .7z, you'd need a third-party tool. Try 7-Zip (Windows) or Unarchiver (macOS), both of which are free.
If this isn't the issue, please check on the creators' websites to make sure the cc items are in fact compatible with the newest patch. If they're outdated, they might not show up in-game at all, or they might cause problems in-game.