This trophy is story related and cannot be missed.
The first thing you need to do is get the post card of Trixie from Cueball and show it to Edna. This will make Trixie so mad that she is willing to help you. She says she needs a photo album of Emmett, a diamond, and some fur. The fur can be gotten from the caveman at the "Hill Valley of the Past" booth, after returning with Emmett's cleanser spray bottle. The photo album can be found in Emmett's Lab after inspecting the easel. The diamond (or prism) can be found at the "Hill Valley of the Future" booth. Use the 3rd and 4th buttons from the right at the same time to knock the prism loose. Once you have all of these items, take them to Trixie to unlock this trophy.
Refer to the Trophy Walkthrough under "To The Expo!" , for more.