There are two different formats for Custom Content: Sims3Packs and Package files.
To get package files into your game, you need to create a new folder yourself, they are not installed through the launcher.
Details can be found here: Install Package Files. To uninstall package files, you can simply delete them and then delete your Cache files (please read special instructions for Injector and object mods)
Unfortunatly great amount of Package files can slow down your game (especially loading times), so combine them to larger files to reduce the lag.
To find faulty package files use the Dashboard tool by Delphy but keep in mind that it doens’t find every kind of bad cc.