Double-click the file and the game will take care of the rest. Gotta love the automatic download. It does take longer than unzipping all your downloads and moving all files at once. The good part is that automatic installs it ensures the files are in the right folders.
The .package files have to be installed manually. That's if you do not have the TS3 Install Helper from Mod The Sims. With this software, you double-click the file and it will install itself automatically.