was designed to not allow this.
What it does allow is for you to put games or applications written by other people. These programs sit on a microSD card. It comes with a 128mb card and a USB reader, as well as a CD containing some games and a video converter used to get your videos in the right format for viewing on the DS. That's right, the Games 'n' Music is a media player as well. It's not the greatest, but it is usable.
There are probably hundreds of games and applications available. Some of them are great, but many of them are not. Below is a relatively boring video demonstrating some of my favorite apps that have some potential in education.
All of the programs I mentioned in the video can be obtained from http://nintendo-ds.dcemu.co.uk/
You will also need a DLDI patcher to make things work. If there's interest I'll create a video demonstrating how to do this.