Catechism of the Catholic Church:
2413 Games of chance (card games, etc.) or wagers are not in themselves contrary to justice. They become morally unacceptable when they deprive someone of what is necessary to provide for his needs and those of others. The passion for gambling risks becoming an enslavement. Unfair wagers and cheating at games constitute grave matter, unless the damage inflicted is so slight that the one who suffers it cannot reasonably consider it significant.
Gambling is not inherently sinful. Its sinfulness would derive from its circumstances. Casinos in the USA are normally pretty well regulated in terms of making sure that the games of chance are not rigged. As long as your job is not directly exploitative of those with gambling addiction then I would say that it is perfectly moral to work in a casino. In essence its no different than being a bartender.