in addition to using winzip/winrar to extract the files, you also have to enable the custom content (the downloads) ingame. when your game is finished loading, a blue box with a list of your downloads will appear. click on "enable custom content" (or something like that - on your language) and restart the game. the downloads won't show up until you restart.
for some downloads you also have to use something called
CEP so that recolors of the thing will show up. (read THROUGHLY before downloading anything!)
also remember to download meshes. otherwise the downloads won't work, and they will often flash blue if you place it somewhere, or not show up at all.
remember to read
everything before downloading something. not all downloads work with every game, and some combinatons (especially hacks) might mess up your game a lot. some downloads also needs certain Expansion Packs (or EP's) to function.
(for future gamehelp, you'll probably get better results if you post in the
game help section. right here, it's a sims stories thread - where people can post their stories from sims, and story assistance. it's better to find the right thread, and read the forum rules before start asking or posting wildly.)