Seeds provide a pause in the life cycle until the conditions are best for seedling germination and growth. The apple seeds must be waiting for some signal to tell them to grow, so look up when apple seeds normally germinate, and provide those conditions (such as passage through cold/winter) to promote germination. It is also possible that the seeds are defective in some way and non-viable, but the first method is probably more likely correct.
Work Step by Step
Think about the role of seeds in the lifecycle: a pause, a wait for favorable times, which has helped make angiosperms very successful. The failure of the apple seeds to germinates indicates a lack of whatever causes the pause to end. Try to imagine where apples grow (there is a usually a cold winter) and how that might relate to the pause/its end.