Test DS Capture page
From SDA Knowledge Base
The Nintendo DS and Nintendo 3DS do not have a video out option which makes recording video footage quite difficult. To get a good recording of video footage you will need to install a modification that allows for video out. This service is available for both the Nintendo DS and Nintendo 3DS.
To record audio you'll need to plug in a stereo cable that has 3.5 mm male plug on both sides. You'll be plugging one end of the cable into the head phone jack and the other end of the cable to the line in jack.