The UUID is available when you launch an *unconfigured, unlicensed* VNS3 Controller instance.
Log in with the default username/password, and go to the "home" page of the instance.
You can reach the home page by clicking on the Cohesive Networks / VNS3 logo on the upper left of the page.
https:/<vns3 public ip>:8000/home
Then you should see something like this: