Um.....Gosh I haven't touched SSBM for i don't know how long, but I think I used the X button...
And since my N64 kicked the bucket and I couldn't get a new one and couldn't play SSB, I managed to get an emulator for it. Since I used the keyboard to play, I always pressed up to jump, but that's a whole different story altogether. Of course, now I'm using my Xbox 360 controller to play my emulators, and I still use up to jump.