Update related to first question, from Wikipedia's "defamation" entry:
For McDonalds to have a claim, McDonalds would have to be able to prove it was damaged by the statements. And to the extent the statements are matters of opinion, they are also protected "because opinions are inherently not falsifiable."
In other words, McDonalds would have to prove that it was damaged by the statements, and also prove that the statements are false. I'm not a lawyer or legal expert, but I would guess both claims would be hard to prove in court.
Interestingly, Wolf of Wall Street did get sued for defamation by Andrew Greene, who "filed a claim saying the character of Nicky Koskoff (played by PJ Byrne) bore a resemblance to himself. He said that the film-makers’ depiction of Koskoff was damaging to him, citing in court papers the character’s portrayal as a 'criminal, drug user, degenerate, depraved and devoid of any morals or ethics.'" I believe the lawsuit is still in process.
Regarding your second question:
McDonalds is the second largest private employer in the United States (behind Wal-Mart), hires about 1 million people per year, and has employed roughly 1 in every 8 American workers. Also, it's widely known that most jobs at McDonalds pay minimum wage or close to it.
With all this in mind, I'd guess that **Jordan chose to refer to working at McDonalds because it's the most widely available work in America and also offers very low pay -- a perfect contrast with how he wanted to portray work at his firm (highly exclusive and highly lucrative).