Plesk and Cpanel handle things very differently and SSL is one of those things. Cpanel requires a dedicated IP address for domains installing proper SSL certificates (non-shared) on them. If you have not already purchased the SSL Basic (or SSL Complete) add-on from HostNed then the button will not even appear.
http://www.hostned.com/optional.htm
Since HostNed is going the Cpanel direction now, the plan details are changing a little bit, but I think the deal is that with Custom Series plans you need the add-on and with SDL-Power accounts it comes with SSL Basic but you probably need to request it through support ticket though.