If you use EA Launcher:

My Documents/Electronic Arts/The Sims 3/Downloads

My Documents/Electronic Arts/The Sims 3/Mods/Packages

If you use CC Magic:

My Documents/Electronic Arts/The Sims 3/Downloads

Most of the time when possible I give you the possibility to install as either Sims3Pack or Package.Pick up a format you rather use (poses can only be installed as Packages), and install it in your game:Place the .sims3pack file in your folderRun the Launcher, click the Download panel on the left and select the files you want to install before pressing the Install button.Note: You do not need to keep the file in this folder, it can be removed safely once installed and the Launcher/game is closed.Place the .package file in your folderIf you do not have or do not know how to set up the Mods folder, I wrote a tutorial here Note: If you have too many files in this folder you should consider merging them or it will make your game load and render slower. Aikea Guinea wrote a tutorial about it, see here This launcher manages both types, so you can place any .sims3pack and .package files you want inFor those who don't know about it, you can find the installation here and a tutorial there