If you've lost your Rock Band 2 manual, you'll need to contact Electronic Arts customer support. As the publisher for that title, EA handles all code issues.
The easiest way to reach them is through their support portal:
Once you've signed into their support portal, you'll have the option of emailing, live chatting, or you can find the phone number for your region.
Please note that Harmonix does not have codes for users who have lost their manuals or need a European/UK code. You will need to contact EA to look into replacement codes.
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