Contrary to popular belief, Harmonix does not have an enormous code vault. Nearly all code related issues are managed by the publisher listed in your game manual, so you will most likely need to contact the publisher to troubleshoot or get a replacement code. The publishers are:
EA: Rock Band 1, Rock Band 2 (Xbox and Wii, and PS3 if you aren’t being able to get the 20 Free DLC), Green Day Rock Band, all Track Packs including AC/DC https://help.ea.com/
Warner Brothers Entertainment: Lego Rock Band http://support.wbgames.com
The only code issues Harmonix can directly address are Playstation 3 Rock Band 2 export if your code isn’t working at the website.