The Serial Number can be found either from Web UI or Console/SSH
From Console/SSH:
vplat# shell
/ # hostname
vserver-v1aa-xxxx-xxxx
Serial Number begins with v1aa
From UI:
Navigate to Servers tab (either from tabs or main splash page)
Click on Server, and then on Manage Certificates.
The Serial Number will be the Common Name of the Built-In Vaion Certificate, (beginning with V1AA)