The lab appears on the map after the major event of the Third Act. The requirements to start this event are on a piece of paper on the wall in the main character's room.
Complete the events of one of the girls in the Waifu House. If you maintain satiety and mood, events will start while talking to her. Also, in between, you need to buy her special gifts and do all of the actions. All these things are done in sequence, one after the other.
In the store. Using the principal's office, you can upgrade the town, go to the Waifu House, or go to the Store. Choosing the store will take you to the shopping menu for the Waifu House girls.
