The serial number for a Nintendo DSi XL is located on the bottom of the system, just below the barcode. The number starts with the letters "WW" and is followed by 9 digits (be sure to include the boxed number at the end).
Has the serial number been removed?
The serial number can also be found in the battery compartment of the Nintendo DSi XL, underneath the rechargeable battery pack. For information on how to open the compartment and remove the battery, click here