You may see a prompt on Ableton.com to enter your 9 digit serial number.
All Focusrite and Novation products that come with Ableton Live Lite use the other, 24 digit configuration.
If you see this please click the "please use the regular registration" link.
You will find your Ableton Live Lite code either in your My Account section, after registering your Bundlecode, or on a card in your original packaging for non Bundlecode products.
If you have any other problems with registration please contact Tech Support.