Marcelo in the Real World may be a mystery story, but the title's not mysterious at all. We learn what it means in the first chapter, when Arturo tells Marcelo he wants him to come to work in the law firm because he needs to learn about the real world. In Arturo's mind, Marcelo's been living a sheltered life at his special school, Paterson, and it's time to integrate him into mainstream society.
The "real world" means different things to different people, though. While Wendell's real world contains yachts and hotties, Jasmine's contains a senile dad and a dilapidated farm. Ixtel's real world is a convent full of nuns, and before that it was the life of an addicted orphan. Arturo's version of the real world is the law firm; Aurora's is children dying of cancer. But for the purposes of the book, "real world" means "not Paterson," and in that case, we're going with Arturo's definition.
Of course, by the end of the novel, Marcelo chooses his own real world—one that's in Vermont with Jasmine. It may not be the real world his dad wanted for him (although hey, at least it's not Paterson), but it'll do just fine for Marcelo.