For all classic modes, selecting Akuma is the same as selecting him in the original version. You must highlight these characters for at least 2 seconds in the following order: Ryu, T. Hawk, Guile, Cammy, and Ryu again. Then, after highlighting Ryu for 2 seconds, press Start and all three punch buttons in quick succession. If you did it correctly, Akuma's portrait will replace Ryu's.