With a laptop, if the sticker is not anywhere obvious, it is probably under the battery. Turn the computer completely off, turn it upside down, and look for one or more latches on the battery. Slide them over, and hopefully the battery will now neatly slide out. Don't force anything.
Hopefully the sticker will be there (it is here to stop it getting scatched and worn so you can't read it during everyday use - it is a good thing, otherwise you will be back here like many people asking us how to read a damaged sticker, a much larger problem).
Write your code down, snap the battery back into place, and turn the computer on.
See here for more details if necessary: http://www.computerhope.com/issues/ch001128.htm