In /includes/admin/configure.php
find
Code:
 // secure webserver for catalog module and/or admin areas?
  define('ENABLE_SSL_CATALOG', 'false');
  define('ENABLE_SSL_ADMIN', 'false');
Alter false to true

In /includes/configure.php
find
Code:
// secure webserver for checkout procedure?
  define('ENABLE_SSL', 'false');
Alter false to true