Instructions
Connect the SD card to your computer using a memory card slot or USB adapter.
Navigate through the file system using Explorer (Windows) or Picker (Mac) and clear the data of any pre-existing data.
Create a folder in the SD card folder and name it "Music," if you want to add music.
Add any desired music files or album folders to the "Music" folder.
Disconnect the SD card from the computer and insert it into the SD slot on your DSi.
Start up the DSi and select the "DSi Camera" application from the DSi's menu. First-time launches of this application will trigger a tutorial on how to use the program.
Select "SD Card" from the upper-right corner of the "Camera" menu after the tutorial. The DSi will begin formatting your SD card to store DSi photo data. This process will not affect the previously created "Music" folder.