download play works on most slot 1 flash carts, with a slot 2 flash cart you will need to flash the receiving ds, there are a few exceptions to this, on game that dont have a data check such as custom robo, bomberman land touch, download play is you sending a mini game to someone who does not have the game, im surprised you are asking such a simple question, when you turn on your ds, the download play button is in between the Start GBA and start ds game slots in the middle of your screen, next to pictochat.