Note that they're only listed there if you've registered the software previously. If it's a new, as yet unregistered purchase then it won't be listed.

Generally the serial number for downloads will be in the email you received. You can also check your order status to find it there. If you ordered the "boxed" version then there will be either a postcard in the package with the serial number or it will be on the first page of the quick start guide, if one was included.