Well this question's really out of date, but if anyone still needs help I can try. :)
1st. Make sure your Downloads folder (which should be located in Users\(Current User Account)\Documents\EA Games\The Sims 2\Downloads.) has a capital "D". So it's Downloads. Spelled just like that.
2nd. Make sure you extracted the files correctly. Once you download them, don't just throw the file into the Downloads folder. You have to extract them using WinRAR or another kind of extracting program. (Download WinRAR here: http://www.rarlabs.com/download.htm)
3rd. Make sure you READ ABOUT THE DOWNLOAD! Make sure you have the correct games and expansions, the right meshes, and so on. Make sure if it says you need to download a mesh from a site, you download it! (It should say if you need a mesh in the "Downloads" tab at modthesims2. If your downloading from xmsims, click on the photo of the download you want, and you'll get the download window.)
4th. If you were downloading a Sim or a house/community lot, make sure you INSTALL IT INTO THE GAME. SIMS AND LOTS DO NOT GO IN THE DOWNLOADS FOLDER LIKE CLOTHES, HAIR, ETC. First save the Sim or Lot to your desktop, or the destination where you first save the downloads before extracting them. Then extract the file/files. Next, double-click on the Sim or Lot. It should say if you want to install them into your game. Click install. (Be sure to have the Sims 2 Clean Installer. It'll install Sim and Lot packages into your game with ease. Download it here: http://sims2pack.modthesims2.com/.
YOU NEED THE SIMS 2 CLEAN INSTALLER FOR SIMS, HOUSES, AND LOTS. If you dont have that, a message may come up that says "Windows cannot open this file." If that message comes up, then you need the Sims 2 Clean Installer!)
5th. Check if your Custom Content is enabled in your game. Go into the neighborhood of your choice, and click on the 3 ...'s in the bottom left hand corner. Then select "Enable Custom Content." (I might not be dead on about the Options menu of the Sims 2, considering I've played the Sims 3 mostly now-days, but somewhere in there it DOES say Enabled Custom Content! I promise! :-) After you've enabled the content, restart your game. YOU MUST RESTART YOUR GAME. :)
6th. Do you know where to look for your content in the game?
If you downloaded hair...:
1. Go into Create A Sim or In a mirror click on Change Appearance.
2. Your custom hair should look like this: http://www.simswiki.info/images/c/c3/ChangeSim1.jpg
With the little star looking thingys on the hair picture.
3. If your hair you downloaded was pink, blue, bright red or some different thing other than the regulars, you might need to click on the little Star Looking Thingy next to the Red hair. Thats for different color hair, and some of your styles might be in there!
1. Go into Create A Sim or go to a community store and click Buy or Try On
2. Your custom clothes should be under Everyday, PJ's, Underwear, Swimwear, Athletic or Outerwear if you have Seasons, considering which kind you downloaded.
3. Make sure you click Outfit if you download an Outfit, and Top if you download a Top, and Pants if, well, you get it. :)
4. Pay in mind not all clothes fit all Sims. Make sure you selected Toddler, Child, Teen, Young Adult or Adult for the correct clothing.
1. Go into Buy Mode.
2. If you downloaded a Bed, click on the Beds. If you downloaded a Dresser, click on the Dressers. Etc...
3. Make sure to click on the little Flower next to the Table in the main categories. Alot of the custom content downloaded goes in one of those, so make sure to check EVERYWHERE.
(If your hair, clothes, or furntiture comes out looking funny, or doesn't show up at all, make sure you downloaded the mesh!)
7th. If you don't know what the mesh is, it's kinda what holds the whole download together. Without the mesh, you get, well, nothing! The mesh can be found either right with the download file, or by it. There will usually be a sign that says "Mesh required, get here." or "Link to Mesh" something like that. Pay in mind not all products need a mesh, so dont freak out if there isn't a mesh there! ;P The product will usually tell you if it needs one or not.
If every single one of these steps didnt help you, I have NO clue what to tell you. Maybe post a question at Modthesims2.com or hey, read this: http://www.modthesims.info/wiki.php?title=Game_Help:Downloading_for_Fracking_Idiots
That's what helped me. If your new to downlaoading content, don't worry, you'll get it. It might not seem simple at first, but it is, trust me. Or if you've been downloading forever and ages, and you just have a problem, posting a question on a forum never hurt! GOOD LUCK AND HAPPY SIMMING!