I know you can go to the Dept of Labor's (DOL) and the Occupational Safety and Health Administration's(OSHA) websites for the rules governing safe working conditions. You can begin by arming yourself with knowledge of the laws and regulations. Most times when you confront the situation, it's enough to cause action on the part of the employer. Unfortunately, I don't know of anywhere you can go to get immediate relief.

McDonald's has a reputation for hiring lazy manager's who really don't know how to manage a business. There is no excuse for not repairing the air conditioning. Maybe you can report it to a city or state agency. Like the Board of Health. You should have one both locally and at the state level.

In the meantime..take extra breaks. Drink lots of water, and make sure you get some extra potassium.

If all else fails, quit. It's McDonald's. You can get a job somewhere else.