To get your 20 Free DLC, type the code from the back of your Rock Band 2 manual into the following form:
Make sure you're typing in the code exactly is at appears on the back of the manuals (sometimes it's easy to confuse certain letters and numbers, such as L and 1 or 3 and E.) Also make sure your email address is spelled correctly.
If you're receiving an error saying that your code has already been used, then either you are trying to register with a used code or you've forgotten that you've already registered! If you'll send us a ticket, we'll be able to confirm when your code was registered and, if it was by you, resend you the email with the download token for the songs.
If you're receiving an error saying your code is invalid, you'll need to contact EA customer support for a replacement. Harmonix doesn't have any replacement registration codes to supply. You can reach EA customer support at:
http://customersupport.ea.com/loginapp/rockband/rockband_portal.html