How do you download custom content on Sims 3?

To put this a slightly different way, CC in TS3 comes in two different formats. Sims3pack files are what we use the Launcher to install by placing them in Downloads. Package files require the Mods\Packages framework, the Launcher cannot read these files. Which format the CC comes in is up to the creator, some of them make both formats available for download so the player can decide which they prefer.

It doesn't matter which file icons the downloads pick up along the way. It does matter which format the files are provided in.

You may also find content, of either type, offered up in compressed files. The macOS can work with zip files just like Windows can, but it requires third party tools like The Unarchiver or Keka to decompress (open up) rar and 7z files (this is where Windows would require WinRAR or 7-zip). In any event, the point is to get the sims3pack or package files out of the compressed download when they are offered this way as nothing in the TS3 user game folder can work directly with zips, rars, or 7zs.

Once you have the actual content files, there's no difference between Mac and Windows as to how they should be handled.

The white text icon is what you place in your mods folder.Do you have a mods folder set up?
If not follow this link: http://modthesims.info/wiki.php?title=Game_Help%3AInstalling_Sims_3_Package_Files%2FSetup_and_Files
Once you got that setup, you plave the white text icon into the mods --- packages folder. Some CC comes as Sims3 packs and those you install just like regular store items.I prefer .package files as they're easier to remove.

I'm not sure if installing mods on macbook is different than windows computers or not.I'm gonna tag @igazor for that.