Moby Dick reflected the dark side of human ambition with captain Ahab pursuing the white whale. How does this symbolize America at that time?
America in the 19th century was not necessarily the America we know today. It was the Westward Expansion, and the paradigm of the American Dream was there stronger than ever in the hopes of finding power and riches. Along the sidelines, Ralph Waldo Emerson's "Self Reliance" was resonating in the psyche of the American culture, with its proposal of building your future and going above and beyond trying (Transcendentalism).
Along with this, Ahab would represent the American citizen of that time, desperate to find its rare, unique, own "great whale" of opportunity to branch out and conquer. There was a deep preoccupation with reaching a successful point, and many were willing to go at any lengths to obtain such successes. Hence, that was the road to the American Dream, and Moby Dick represented this turbulent and hard task in the allegorical representation of this evading, yet so evidently present whale.
Moby Dick was published in 1851. You could certainly make the argment that the United States reflected the dark side of human ambition at the time.
Think about the sorts of things the United States was involved with at that time. The US had just fought a war with Mexico (1846-8) that was mainly meant to take land away from Mexico. The US was in the ongoing process of taking the West away from the Indians. Above all, the US was at a point in its history where much of its wealth was being created by the use of slaves.
So, you can argue, as Meliville did, that the US represented the dark side of human ambition at that time.