The Question and Answer section for Death of a Salesman is a great resource to ask questions, find answers, and discuss the novel.
Before learning that his father had a woman on the side, Biff was completely into sports and idolized his father. Walking into the hotel room in Boston changed everything.
Willy's American Dream is to be successful...... and well liked. He believes that success is directly dependent upon being well liked. As a salesman, he believed that being popular was integral to making sales and growing a customer base.
I don't know that anybody in the story actually sees the American Dream for what it really is. Most characters, like Willy and Biff, grasp at the illusion without fully comprehending it.