Go to a S.H.I.E.L.D. save zone and select "Change Team". Delete all selected characters by pressing Square. Once all of your team slots are empty, quickly press Down, Left(2), Up, Right, Up, Down, Left, Start to unlock Silver Surfer. If you entered the code correctly, you will hear a sound. Note: This code may also be enabled at the main menu. Alternately, complete all Comic missions.