I've been looking for my serial number and I couldnt find it. I do not know why but my DS doesnt have a white sticker with the barcode along with the serial number(I didnt rip it off). Plus, I threw away the DS box a when I first got it. P.S: I got the DS when it first came out.
On nintendo.com, it says the serial # is on the bottom of the System. It is the set of #s and letters under the bar code.