Re: Sims 4 work expansion: This Code has already been used

You have already registered the game yourself and are trying to reinstall the game

The game does not need to be registered again. Login in the Origin Client with the Email/PW you first registered the game on and download and install the game from there. The game only needs to be registered once and you won't need the code again after your initial registration. This is true for both the DVD and the download version. You can skip the activation step on reinstalling.

Source: http://answers.ea.com/t5/The-Sims-4/FAQ-Serial-Codes-Game-Activation-and-Download-Issues/td-p/4228232

And: 
All on the Sims 4 will go under one picture in the 'My games' tab in Origin, try this:
- Go to the 'my games' tab in Origin.
- Hover over your 'The Sims 4'.
You'll see a little screen come up with an 'i' and a 'play/download/install' button.
- Click on the 'i', you'll get a different screen.
- Scroll a bit down and you'll see your expansions and other stuff.