Listen, it's all about practice.
I personally think the dpad is the way to go, but most people prefer the analogue stick. You gotta do one fluid motion from down to forward; that is you have to hit down, down forward and forward and then hit the punch button for a fireball to come out.
Practice simple fireballs first.
Then move on to the Super motion; that is you gotta hit the the fireball motion twice before hitting the punch button.
I can write about it all day long but you have to practice it to make it work. So get down to it!