Digimon World: Dawn and Dusk/Find Tsukaimon

To start the quest, go to Dark Office, enter Dark Center, and talk to Myotismon at the Dark Species Quest Counter. From there, go and talk to Pagumon at Dark Plaza.

Finding Tsukaimon
Tsukaimon can be found in Thriller Ruins. When you first enter, go up the ladder closest to you and open the chest. Go back down and walk towards the exit at the northeast corner of your current location, then go north until you hit the wall. Keep going east until you see a ladder. Go up the ladder, open up the chest on the leftmost part of the platform, then go and talk to Tsukaimon, who will tell you that he will return to the city.

Backtrack your way to the entrance of Thriller Ruins. (Alternate routes are still blocked off at this point.) Return to Pagumon in Dark Plaza and he will tell you to return to Dark Center for your reward from Myotismon.