I have two systems. Both 3DS XLs. The blue one we've had since Christmas, the Red we just got tonight. I know that the Download Play works on the blue unit as we have connected it to a friends unit and played with no issues.
The red 3DS has internet. I have connected to eShop and downloaded Swapnote. I have been able to connect to the blue unit and transfer MIIs back and forth.
I have updated the red units system as well.
I can't get the two units to connect using Download Play.
The game I am using is New Super Mario Bros.2.
HELP!