Try looking on the box, or check your emails sent from Adobe, and the serial number should appear there. If not, ring support, and give full details of the order, and they will send you the serial number again.
If, however you have an illegal version of adobe, or a trial version of adobe, it is against all laws, both national and international, to get a serial number for adobe, unless you have purchased the product (try looking on Amazon for some good deals for adobe)