That's the serial number. If that is missing, look on the 3DS. The pin number is on a small packet in the box of the Nintendo 3DS, with Club Nintendo on it.
You can still register the serial number to get the 90 day warranty extension. If you want the coins, however, you will need that PIN#. If you really lost it, maybe Nintendo can help you out. You can call them at 1-800-255-3700.