To unlock Akuma you must first beat all fighters leading up to M. Bison without continuing and in under 20 minutes - no easy feat. When you fight M. Bison Akuma will appear and you'll end up taking him on. You don't need to beat him to unlock him. After his appearance, you can highlight Guile on Arcade Mode's character select screen and press Up to select Akuma. In Vs. Mode you must select E. Honda and press up to select Akuma.