If the cert has been installed correctly then you need to edit the
/includes/configure.php and the /admin/includes/configure.php file.

You need to change the URL to read for HTTPS_SERVER to your url with the https and no www.

Then down lower change the line to:

define(ENABLE_SSL_ADMIN', "true'

Keep a copy of both files before you edit them. In case you do something wrong. You just need to get the https://your doman name url correct and enable SSL from false to true.

Then the lock should show up.

Ray