As of today, Seasons is available and EA has major issues. It seems that they have Server Crashing issues and because of this, all registered games have disappeared from our MyPage again.
There’s nothing you need to do about it. EA is working on it.
If you bought the download version through Origin and your accounts are connected, there’s no need to register the game. It should be automatically added to your thesims.com account (but since that does not work correctly, it won’t show on your myPage.
Try redeeming your limited edition codes anyways to see if it works.
The links can be found here: How to redeem the limited edition content
Download not showing up
If your Download is still not available in Origin, try logging out and back in and if it still does not show up, you probably need to wait. If the servers are crashing, there’s nothing you can do about it.