The Sims 2
Downloaded .package files go in this directory:
My Documents/EA GAMES/The Sims 2/Downloads (If you do not have a 'Downloads' folder, create one).
Downloaded .sims2pack files:
Double click on the .sims2pack file and a download installer will appear.
The Sims 3
Downloaded .package files go in this directory:
Program Files/Electronic Arts/The Sims 3/Mods/Packages (If you do not have a Mods/Packages directory, create one).
Downloaded .sims3pack files:
Double click on the .sims3pack file and The Sims 3 Launcher window will appear and install your content. (Some people have trouble using .sims3pack files with the 1.8/2.3 patch. So, either use (if applicable) the .package version. Or, download the latest patch.