The serial number would be in the documentaion that came with it.
There may be a pdf file. Check for a any number begnning with M and follwed by 10 digits.
Or.
Direct no for Symantec Licensing
Call on 1800 721 3934 and select option 2.
Give the details of your company and then they can help you with the serial no
the best and faster way to get the License is contact Symantec Licensing Department for the same.
Website: http://symantec.custhelp.com
Phone number: 1-800-721-3934
Email: license@symantec.com