The first thing I would do is reinstall Origin. There are a few instances where the install may become corrupt.
After you uninstall Origin, you can find the most up to date version here:
http://www.origin.com/download
After you reinstall, does the game appear in your games list? If not, try the code again.
If it still does not work, I would recommend contacting EA directly at the link below:
https://help.ea.com/contact-us
Type in a brief description of the problem you're having (ie. game not appearing in Origin) and then click on "Live Chat" at the bottom of the page. It may take some time before you get a representative due to issues with the SimCity launch but they will get to you.
