Interpol is not the police agency for common people. It is the police agency for the polices. So enforcement agencies from around the world, report any international crime to Interpol. So it is impossible for you to report to Interpol in any way.

If you have solid and documented proof that any particular casinos is cheating people, you can get in touch with the Cyber Crime department of your local police station. If they find any instance of international crime after investigation, they will report it to Interpol if necessary. You don't have to do anything there! But make sure you are not taking to the air - you need to have solid proof of cheating!