Custom Content or CC is placed in The Sims 2/Downloads folder, but the Location is different depending on if you have the Disc version of The Sims 2 or The Sims 2:Super Collection.
Note: on Macs the ~ means Users Home Folder. So in the directions you see the ~ character it means that the following Folder is contained in the User's Home Folder what ever it might be named.
For the Disc version it is easy to find and it is ~/Documents/EA Games/The Sims 2/Downloads
If the Downloads folder is not there then create a folder with the Name "Downloads" with out the quotes and then see Step 2 below.
If you have The Sims 2:Super Collection like I do then the proper folder is not easy to find, it is Located here: ~/Library/Containers/com.aspyr.sims2.appstore/Data/Library/Application Support/Aspyr/The Sims 2/Downloads
To get to the proper directory you press cmd+shift+G or go to the Finder->Go Menu->Go to Folder and then type this in to the box:
~/Library/Containers/com.aspyr.sims2.appstore/Data/Library/Application Support/Aspyr/The Sims 2
Next when you are in The Sims folder make a new folder with the name Downloads
Next make a subfolder inside the Downloads folder and name it Mods for the Mods that you download and make a subfolder named Clothes for the clothes and the same for Hair, Vehicles, etc.
Keeping things inside of subfolders is a very good idea it makes it a lot easier to find something when you need to find it or test it, I also organize by Creator so for example /Downloads/Mods/InSIMenator.
Extract the downloaded files and place them in the proper Directory, most go into your Downloads folder, but look at the directions if there is one, i.e. Terrains would go into the SC4Terrains folder.
Downloaded files are usually compressed and in .RAR or .ZIP or .7z formats and the best solution for Mac users is The Unarchiver which is free found on this page http://unarchiver.c3.cx
However Sims2Packs require you to use The Sims™ 2: Package Installer, which you need to download from Aspyr if you haven't already.
Note: The Clean Installer is PC only.
Note: Lots and Sims are in .Sims2Pack format and if unfortunately the Lot or the Sim was created with anything newer than Bon Voyage like Free Time or Apartment Life it will not work and you will get an error message.
After you extract the file, you should have a folder with files or a file with a name ending with .package, like:
These .package files you move into one of the subfolders in the Downloads folder depending on what kind it is clothes go into the clothes subfolder etc., if it is a folder move the entire into one the proper sub folders. If you don't have a sub folder for that type of item then make one first.
You can delete the .RAR, .Zip or .7z file, but it is wise to save these files in a back-up folder in case something goes wrong.
Me I backup my entire The Sims 2 folder onto a different drive, in case something goes wrong. Word to the Wise Make Backups!