Your Design Opportunity is one or two paragraphs that describe what you've been tasked to create, who it's for, and what the parameters are. In this case, you'll want to describe your theme and what that theme is about. You can give some information about the materials and equipment available to you and the time in which you'll be allowed to design and create. The size parameters are especially important here, but you do NOT need to state exactly what you'll be making.
Your Design Brief is one or two paragraphs that describe specifically what you will design and build. Explain the function of your piece, its theme, and how it will embody that theme. Give specifics about materials and sizes as best you can. Since some part of your piece must be digitally fabricated, you could explain how you'll work that aspect in.