you have to do it through linux, no way to do it through zentyal, zentyal uses bad certs, so the only way to to fix is to get it signed and manually add ssl through terminal...same way you would do it stock linux.
Edit: I read the link, I mean try it and report back? Its ubuntu linux so there is a TON of documentation out there. Its standard ssl cert, I have mine signed by a third party to get rid of the security error on the sites and services I host.