Yea, it's only the high-flyer that can do it. I don't know about the PS2, but for PC, it's freestyle superstar button plus the dunk button that you should tap. And if there's a short enough defender at the right possition it'll work.
Perhaps you should go into 1on1 with youself and place a short player under the hoop and try it yourself till you get it to work.
By the way, next time wait longer than 12 minutes before bumping a topic (edit it instead if it's that little time in between).