Where do I find the registration code for Sims 3 Late Night Expansion Pack?

Look at this link to learn more about replacing codes: http://answers.ea.com/t5/The-Sims-3/FAQ-Lost-and-broken-DVDs-and-missing-serial-codes-What-to-do/td-p/14230

Your registration code should be on the back page of the manual.  Origin/EA will only help you replace a code  (or tell you your old one) if you purchased the game from them or already registered it with them and then lost the disc for example.  If you only have the disc but do not know the code, they will not give you another.