Shared SSL is a common certified SSL available to all of the sites on a server.
Your shared SSL address is your host server name followed by /~YourUsername preceeded by https://
Note the "s" in the above prefix. If you don't include the "s" in the address then you will not be visiting a secure version of your site.
If you have an Addon Domains that you would like to use the shared SSL on, you must first determine the name of the subdirectory in which the Addon Domain resides. To determine that folder, login to the cPanel for your hosting package, commonly found at http://www.yourdomainname.com/cpanel (yourdomainname.com should be replaced with your actual domain name).
Click on the Addon Domains module.
Scroll to the bottom of the page to the list of your Addon Domains. Look at the Document Root for the domain name in question. It will probably look something like this:
In which case, the shared SSL address for that site would be: