This article will go into detail on how to install certificates on OS X El Capitan.
In the Finder window, under Favorites, click Applications, expand Utilities, and then double-click Keychain Access.
Keep this folder open so that you can readily access this file.
In the Finder window, under Favorites, click Applications and then double-click Server.
For example, in the Certificate drop-down list for Websites (Server Website – SSL) select your new SSL Certificate.