The hostingaccount that you add in WHM are still subject to at least 48 hours propagation time. That means, after you point the domain to the correct nameservers, it will take 48 hours to begin to work. You should point your domain to the name servers as given to you. This change in name servers can be done form the domain administration control panel as provided to you by your domain registrar.
After name servers are propagated your clients can access cPanel hosting control panel from http://theirdomain.com/cpanel