Make sure that the original owner didn't already register the base game - that's probably the cause of your problems. A game cannot be registered more than once.
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I will most likely not see your response. If no one responds to you within a few days, contact EA here: https://help.ea.com/contact-us/ask . For help with their website see this: http://sims3.crinrict.com/en/2012/10/how-to-contact-ea.html. It will probably be best. If I did help you and solved your problem, please accept the solution (XP is also nice). I hope it all works out for you - happy Simming!