Adding a dedicated IP address and SSL certificate to Plesk 11 by default doesn't work, because nginx doesn't work properly and causes 404errors amongst other things.
Procedure to get it to work/install is :
Tools/Settings IP address - if the IP you want to use is there, delete it. Then re-add it.
Tools/Settings SSL certificates - add your SSL certificate. Note: Plesk needs a long 2-part CA certificate otherwise it won't register it correctly.
Tools/Settings IP address - choose the dedicated IP and select the SSL certificate you just added.
Tools/Settings -> Server Management - restart Apache. restart nginx.
Go to your Domains -> click your domain and then Web Hosting Access. Change the IP to the dedicated one.
below the options click the actual domain name itself and Enable SSL. Do not worry about the certificate, leave it as "Not Selected" - stupid Plesk seems to ignore that anyway.
Back to - Tools/Settings -> Server Management - STOP nginx. Restart Apache.
If you don't STOP nginx - you'll get a 404 error.
Try to reach the website in your browser. Then do the same with an https:// secure address. Hopefully works again.
Once more Tools/Settings -> Server Management - Start nginx.