Go to the TV station in Jubilife. Talk to the guys, i think, on the 3rd floor that is
standing next to a girl. The first question they ask, reply, "Everyone happy" and then
next say "Wifi Connection." Tell me if that helps.

Go To Jublife City go to the Jublife TV station then go to the 3rd floor go down to the
little room on the 3rd floor then he will ask you to say somthing about the tv or somthing
then put in everyone happy (go to abc mode in both)then he'll ask for a feed back and put
in wi-fi connection then save the game turn it off and back on and wala at the menu under
new game it will say mystery gift (you need to ds's that both have mystery gift) GOOD LUCK

sorry i ment how do you recive it

i had the same problem, i got the 'mystery gift' then nothing happening, ive got it but
cant recieve it?? :S

You have to go to a nintendo event. Also you can get them over wifi.