When you download a Package file from the web, it would be compressed as a Rar file (Its extension is ".rar") that you have to extract using WinRar by right-clicking on the file and choosing "Extract Here", a Package file (Its extension is ".Package") should appear, here's the way to install Package files into The Sims 3:
1- Go to your game folder which you run the game from (The one that has the "Caches","Extras","Game","Game Data","Support" and "Thumbnails" folders), create a new folder in it and name it "Mods", enter the Mods folder and create another new folder in it and name it "Packages". put the Package files you have in the Packages folder.
2- I uploaded this file for you since my last source for it has expired, Download it from here:
http://www.mediafire.com/?fehpah75n1k3fwr
it's called "Resource.cfg", just put it in the game folder.
3- Done, start your game and you'll find whatever Packages files you put in the Packages folder installed in your game.
Note: If you don't know where the game folder is, its default location is "C:\Program Files\Electronic Arts\The Sims 3", but if you chose the game folder location manually when you were installing the game, it would be where you chose it to be. I'm an XP user so I'm not sure of the location of the "Program Files" folder, but I think you know where it is in Windows Vista.
If you face any problems, feel free to e-mail me from my profile page. you may encounter ".Sims3Pack" files that have another way to install, but forget about them till you need to install them.
Oh! and the answer above me should solve the problem for you too.