In the intro/tutorial level, takes 10 minutes or so.
After the memory is spotted, go back up the way you came, but grab the blue battery as you go. Take the middle route of the 3 way pencil bridge back past the house, then down to the floating platform that recently started moving back and forth. When you install the blue battery, 2 new pencil bridges will appear: one to get you to the memory and the other to get you back to progressing the story.