what does biff mean when he said that his father had the wrong dreams?

 Biff is actually referring to the fact that Willy was following another man's dream. When Willy was younger he was infatuated with a salesman, Dave Singleman, who apparently made a lot of money out of a hotel room. Willy wanted to do the same. He became obsessed with easy money and the American Dream. Biff points out that Willy actually wanted to build things but instead chose the wrong dream.