You can install a shared server certificate under a domain under the user level of the Admin login. If you don't have a domain there, you can create one, and after the Certificate is installed, you can remove the domain without deleting the Certificate, but if you do you'll need to recreate it. The domain name can be anything, but you'll need to follow instructions at PositiveSSL site to add domains to the Certificate.
What does the popup notice say?
Jeff