Logging into cPanel is very easy. All you have to do is go to Domains -> Domain Groups -> Select the domain -> Click "Actions" and click "Login to cPanel" under the Hosting account section.
Another way would be to go to Hosting -> Choose your Hosting account -> Click on "Login to cPanel" under the Service Details section.
You cannot access cPanel over yourdomain.com/cpanel because it's disabled for security reasons.