This will be unlocked in episode 4. I would suggest just following the walkthrough here on TA:Back to the Future: The Game - 30th Anniversary Edition Walkthrough
Here is the description coming from the gamers who created the walkthrough (Credit to them of course):
Talk with Doc to learn he's getting cold feet. When Doc asks how Edna ends up, he won't be satisfied with any answer but the truth - that she's alone and miserable. Hearing this leads Doc to have a change of heart and regardless of what you say, he leaves with the DeLorean. Provided you followed all the other steps, Emmett should arrive. If you missed the Let Them Eat, Um, Cake achievement, this is your last chance to offer a cake (to anyone except Doc of course) and pop it now. When ready, use the can of oil on Emmett again. Emmett inadvertently destroys his suit, Trixie plays her part perfectly, and when Emmett tries to exonerate himself with his invention, your sabotage foils him. Wow. I hope you're proud of yourself.