Assuming you mean on a V4 or V4.5... the matchmaker should visit a couple of days after your Tamagotchi character becomes an adult. Note: if you pause it for long periods of time (such as during the day, while at school,) it will take longer before the matchmaker begins visiting.
With my tamagotchi version 4 I found that the matchmaker would usually come when my tamagotchi was at least 5 days old. I also found that the matchmaker would come at certain times. With all of my tamagotchis she would come at 1pm, 3pm and 5pm. Try not to pause your tama too much and try remember back to when the matchmaker would usually come for your other adult tamagotchis.