Wait until evening, then go into the kitchen, open the refrigerator. Inside there are three boxes: one for Molly, one for Jenny, and a black box. The black box contains the key. Use that key to open the vent. This game is great and it has no bugs, haha.
Hmmm I don't know. Perhaps you don't meet some of the requirements? Or you can click on the question mark in the upper right corner of the screen to see the instructions
(I unlocked the vent event. First, I had to tap on the vent when I saw the glowing eye inside, then talk to Molly, Jenny, and Amy about how to unlock vent. Wait until night and then I got the key from the black box in the refrigerator)
Try it again.