If some of your .package files are working in game then it means you're installing them properly. If only some stuff doesn't show up, but others do, then it's possible there's something wrong with the items you're trying to install. Also make sure you don't have the old way still in place (the .dll file in Program Files and so forth) as that can cause issues.
.Sim files get put in My Documents\Electronic Arts\The Sims 3\SavedSims\ - if you don't have a folder called that, make one there. Call it EXACTLY that. "SavedSims" - two capital S's, one small s at the end. You'll find them in Create a Sim in the premade sims bin (big button, down at the bottom, looks like silhouettes of people's heads). You'll need to make sure you're looking at the right age/gender for the sims you've downloaded. You can click the * button to show only custom sims.
Put accessories on sims in Create-a-Sim (it's done per outfit) to do it on a new sim, or send a sim to a dresser and plan them a new outfit.
Objects show up in-game in the same place as any other object - in the buy/build mode catalog. Custom items installed the new way in My Documents\Electronic Arts\The Sims 3\Mods\Packages will have a little icon on them to indicate they're custom items.
If your game is patched to 1.12 or higher, the "framework" required to install cc is simply
• the Mods folder you place in Documents/Electronic Arts/The Sims 3/,
• the Packages folder you place inside the Mods folder, and
• the Resource.cfg file you place inside the Mods folder (download from Step 2 here)
Place any .package files inside the Packages folder.
Place .sim files in the SavedSims folder as described by HP.
Install Sims3Packs via the launcher.
I realize there's a lot of conflicting information floating around about how to install TS3 cc, but that's just because until patch 1.12, EA had not made the game cc-friendly. If you had cc in the game prior to patch 1.12/Ambitions, then you need to remove any cc installed in the old location.
Don't use the monkey helper. It's designed to place .package files in the Program Files path, which is the outdated method of installing cc. patch your game to 1.12 or higher and use the more recent method for installing cc, described above.
Visit those FAQs, and you will see screen shots. If you still have questions after that, please do what the introductory message asked:
If your .package format custom content is not showing up and you have followed the above FAQs: Please see CC Not showing? Read this!!! You MUST provide the requested information there when posting a Help thread about your custom content!
Copy and paste the indicated line from the skuversion.txt or info.plist from the following locations, for whichever games you have (adapt as appropriate for OSX):
C:\Program Files\Electronic Arts\The Sims 3\Game\Bin
C:\Program Files\Electronic Arts\The Sims 3 <EP or SP name>\Game\Bin
... and attach them to a post on this thread. Click Go Advanced and then Manage Attachments. Or if it says the pictures are too big, you can upload them to modyourpanties.com and copy and paste the links.
We have a guide on how to take screenshots if you need assistance: Game_Help:Screenshotswiki.
Make sure you set your folder view to Details mode and expand out the Name and Type columns so they're fully readable.
Open your Documents\Electronic Arts\The Sims 3\Mods\resource.cfg file in Notepad and copy-paste its contents here.
Right-click on Resource.cfg and check Properties. Is the file name Resource.cfg or Resource.cfg.txt or something else?
Don't mess with the .dll files. You don't need to bother with them at all if your game is fully patched. Messing with .dll files was only necessary before the 1.12 patch - if you're on 1.18 it's irrelevant to you, and installing a .dll with 1.18 can actually make some mods -stop- working.