I got it, it came with the US version of SF Anniversary for PS2.

To play as Akuma, hightlight the following characters, in turn, waiting one second between each selection: Ryu, T. Hawk, Guile, Cammy, Ryu

When you get back to Ryu press Start and all three punch buttons within one second after pressing start to get Akuma.

But be warned, Akuma sucks in this game, no supermoves at all.