And the Sims2Pack files don't get extracted to your Downloads folder. Any Sims2Pack files you see in your Downloads folder (not .package files...those ones stay in your Downloads) should be moved elsewhere, like your desktop, then double click on them to install them in your game. I would recommend you get
Clean Installer to use as your default program to use, rather than use the maxis program. The main reason I suggest this is it will help you install sims that have been created with an ep you don't have (something the default program won't let you do). Also, sometimes a mod/hack may unintentionally get packaged with a house. When you double click on the Sims2Pack, all the items included in the package will show up in a list. You can pick and choose what from the package you want to install. Hacks show up in red. If you see them, you can unselect it and it won't be put in your game.