If you want the forum to work with https, first you need to set a custom domain for your forum, with the same base domain as the site you're embedding on. For example, forum.magnet4water.com. You'll need to set a CNAME record that points to n8.nabble.com.
more information:
http://magnet.80003.n8.nabble.com/help/DNSConfiguration.jtpOnce you've done that, I'll be able to enable https on our end.