The Matchmaker will come 72 hours after your Tamagotchi evolves into an adult. What I mean is that she won't come immediately, she will come at certain times during the day once your Tamagotchi has passed that age.
She will visit daily at 10.30am, 3pm and 7pm until your Tamagotchi either gets married or evolves into an oldie.
On the earlier batches of v4, the Matchmaker will only stay on screen for 10 seconds. Literally. Apparently this glitch was fixed on some of the later batches and the v4.5.
To avoid missing her, set the time to either 10.29am, 2.59pm or 6.59pm and wait a minute. If the matchmaker doesn't come, your Tamagotchi isn't old enough. If she does, press the B button as fast as you can to avoid missing out on a mate!