How do you get a mystery gift in Pokemon diamond?

Answer 1

first go to jubilife city and go on the third floor and talk to the guy next to the girl hes going to ask you a stuff just say happy first then wi-fi connection and turn off the game after saving then go on the menu and there will be the mystery gift option then click on the second option and after waiting go to a poke mart then talk to a green guy in the mart and he might give you a mystery gift

Answer 2

go to the 3rd floor at jublife tv. talk to the man and answer his questions with these words. everyone happy and wifi connection then save the game