Sims3Pack file goes into Documents\Electronic Arts\The Sims 3\Downloads, and then you open the launcher's Downloads tab and install the skin from there. What you do next depends on the type of file you wind up with. zip file or WinZip > Extract for the others. In any case, right-click on the file to show the options, for example Extract for a.
7z files do need an outside tool such as WinZip. zip file, you can extract it with Windows.
