Well,firstly,when I run the game a notification in blue background appears,saying that lately custom content was always unabled and If I want downloaded objects to show up in the sims 2 I have to choose to enable It and then restart whole game.Seems pretty easy,but It isn't,because when I return to the game the same notification appears with the same requirements...I don't get It. Why I can't save new options? Even in settings menu I can't change something,because It doesn't stick there after I reload game...
Any advice? I want to add some nursery stuff and decor so badly!

Do you have any CC in your Downloads folder already?
You have to make sure you do save it as enabled and actually have some CC to make it show.

If you have custom content, a dialog will automatically appear when you start up the game. It lists all the custom content that has been loaded in your game, as well as whether custom content is Enabled or Disabled in your game. At the bottom, there should be two check boxes - one is "Disable Custom Content", and the other is "Enable Custom Content". Click on "Enable Custom Content", and then you must restart your game, by closing it and then re-opening it.

Once you've done that, the dialog should re-appear again. The check boxes at the bottom should still be ticked at "Enable Custom Content". If that's the case, then custom content should appear in your game, provided you placed something in your Downloads folder.

For more information, you can see this article.

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Yeah,I've done that before millions of times,but it doesn't worked for me because I can't even save options.They won't last. After I restart game it still shows that I'm disabled cc. I searched everywhere for information but haven't found anything familiar...

You'll need to make sure that you are the owner of your Docs/EA Games folder (in Properties -> Permissions), and it's also a good idea to disable that popup in the game options, since often CC won't work if that message appears even if the "Enable" tickbox is checked by default.

I'm having the exact same problem. I'm running bootcamp on a Mac don't know if that's an issue. But I've done everything... Downloads folder in the directory (Origin Games > The sims 2 Ultimate Collection), enabled the CC on the game, restarted it dozen of times... and nothing.

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God of the Sims hates us for sure.