An allegory is often a tale that has figurative representations within it that symbolize greater things, stories, or morals. So, in "The Rocking-Horse Winner," consider the whispers of the house, and the small rocking-horse itself. These objects and events symbolize greed, and how people are fixated with obtaining it, and yet how the getting of it is futile. No matter how much money Paul wins, the house still whispers for more money. Greed is an ever-hungry demon that needs constant feeding and is never satisfied. Paul seeks to obtain money on a rocking-horse; this symbolizes the futility of his search. A rocking-horse never goes anywhere; it just is a semblence of reality, of the real thing. This is the same as money--people think that money will bring them happiness. In fact, they feel it IS happiness. However, like the rocking-horse, it is just a facade, a semblence of true happiness. Real happiness does not come through money. It comes through love, family relationships, and self-worth. That is what Paul truly seeks--his mother's love, and a sense of self worth. Seeking money to find it never works.
In these senses, objects and events within the story itself are symbolic of a greater theme and message, and that is what makes it allegorical. I hope that those thoughts help; good luck!