How does the motif of dreams vs. reality support the complexity of the play?
How does it "support the complexity?" What does it mean to support complexity? This is a strange assignment.
Well, when that happens, just do your best to interpret the teacher's meaning. Or ask for clarification. But in this case, you can start by writing about that motif.
How about starting by reading this great analysis: .sparknotes.com/shakespeare/msnd/themes.html
Start from there. Enjoy it! The point is to write a few paragraphs about the nature of dreams and the nature of reality. What lesson was Shakespeare teaching with this motif?
Just start by writing about that. After you have written something, you can discuss its complexity. :-)