See Trivia Manipulator for details on how to start the Horseman gathering quest.
Go into Bosco's Inconvenience and ask Bosco about his package. He'll reveal that his mom sends him a bomb every year and he's not sure if he can hear the bomb ticking or not. Head back to the North Pole and go into Santa's Workshop. Head to the green computer and use it to dispense a "Obsess-o-matic stopwatch". Pick up the gift tags on the counter next to the present, and then use the gift tags on the present. Send the present to Bosco. Then head back to Straight Street and go back into Bosco's Inconvenience, where Max will automatically open Bosco's original package and take the Horseman that's inside.