I don't know if others have already discovered this, but I'll just tell my story.

I've been playing around with DSiPaint on my 3DS, because it didn't work. But then suddenly I figured out how to make it work!

It does not work very well, but there are also some advantages on a 3DS.

So here is how it works:
- Go to DSiPaint.com on your 3DS and log in
- Click on DSiPaint and open it
- Pick a color
- Click and hold your stylus on the same spot of the canvas
- When an arrow appeared, keep holding down your stylus and draw something (for example a line)
- The canvas should still be empty now. Click the zoom in button, and after that the zoom out button. Your line has now appeared

This quite gives hope for a 3DSPaint. Although you have to click and hold always, the line you draw is much smoother than on DSi's. That the line only appears after zooming can maybe be fixed, and the saving too (this doesn't work, it will save a broken image in your album)

The future will tell! Maybe I'll also do some research myself, although I'll have to learn how canvas works.