The matchmaker will begin visiting after your character has been an adult for 3 days (72 hours) unpaused time.
The visiting times are 10:30 AM, 3:00 PM, and 7:00 PM. If you've missed her, you can set your Tamagotchi's clock to 1 minute before one of these times, then wait for her to appear. (Don't forget to set the clock back to the correct time afterwards.)