Certain items, such as the Flash DLC from the Collector's Edition or promotional codes (e.g. WWE), can be obtained by entering your redemption code through the in-game menu under Extras > Redeem Code.
Please note that all codes need to be entered exactly as printed. That means that you need to include dashes (-) and avoid adding extra spaces, at the beginning or end of the code. Be sure to enter the code correctly to prevent any errors in the entry system.