In The Sims 4, there are two custom content folders: Tray for lots and Sims, and Mods for other content. Both of these folders are located in:

Windows XP: Documents and Settings\(Your User Account)\My Documents\Electronic Arts\The Sims 4\

Windows Vista, Windows 7/8, Mac: Users\(Your User Account)\Documents\Electronic Arts\The Sims 4\

When you download Custom content for your game, this comes in .Package files.

All custom content in Package files need to be placed in the Sims 4 Mods folder. 

The Mods folder may contain sub-folders. If the Mods folder does not exist, it can be created manually. The Mods folder will contain a file named resource.cfg . If it is deleted, the game should recreate it.