Compatible Music Formats
The only music format compatible with your DSi is ACC, with the file extensions of M4A, MP4 or 3GP. Even when formatted appropriately your music still may not work on the DSi; any music with DRM protection will not play on the DSi. You can format your music to AAC format using a couple different programs. Nintendo recommends using Nero's Music2Go, an inexpensive commercial program designed to format sound files for the DSi. You can also use iTunes to convert songs to AAC free (see Resources).
SD Card Specifics
The DSi has one SD card slot, though you can also use other cards that have an SD card adapter. The system is compatible with both SD and SDHC cards, with capacities ranging between 2GB and 32GB. Cards with higher speeds will work better than cards that work at lower speeds.
Adding Music to Your SD Card
Insert the card into your computer; many computers and laptops have an embedded SD card reader, though USB card readers can be purchased at any major retailer. Open the card in your file manager; Nintendo officially supports PCs, which use Windows Explorer, and Macs, which use Finder. Copy your music files or directories onto your SD card; there is no specific folder where you need to copy your music files.
Listening to Music
Click the Nintendo DSi Sound icon on the main DSi menu, which looks like a bird next to a music note. From there you can select the option to play your music from the SD card. You can select a specific file to play, or start with a random song by clicking "Auto" then "Start."