Re: where is my social code, pls

@HuNingning  The code you need is the CD key for the base game, although you can also register the keys for any packs you own and get some bonus content.  Open your Steam library, click on Sims 3, then the gear icon (in the red box below), then Manage > CD Keys, and you'll see a key for the base game and each pack.

On the Sims 3 site, you can register any key you want by clicking "Register a Game" at the bottom of the screen.

You may also get some SimPoints for registering the base game, and for World Adventures if you own it, although I don't know whether this offer is still valid.  If it is, there should be a button to redeem your points on the left side of the page, I think above the list of packs you own.