Flat characters are characters that have one or two characteristics and are not fully developed, whereas round characters are ones that are developed and we are allowed to see them as complete characters. Going by this definition, it is clear that both General Zaroff and Rainsford are round characters - they are characters who are presented to us in completeness - we are allowed as readers to understand what is going on inside of their heads, especially Rainsford. Although we get little information about Zaroff's internal workings, we are shown his motivation for setting up the "hunt" or the "game" that dominates the end of this story.
You might like to think about some of the other characters to enable you to see the contrast. It is clear that Ivan, the Cossack servant of General Zaroff, is a flat character - we have little access to his thoughts and he really only serves one purpose which is to look big and menacing and serve Zaroff.
Hope this helps you understand the difference a bit better!