See Screenshots here: http://imgur.com/a/7TOk1
From you Billing Panel, proceed with the following steps:-
From the "Services" menu, click on My Services
This will take you to the "My Products and Services " Page
On "My Products and Services ", Click on your "Active" Green button.
This will take you to a new page, Under "Actions " Section (See Left Side)
Click on "Login to cPanel " on the left page panel of the page.
See detailed visual instruction here: http://imgur.com/a/7TOk1/all
Alternatively, you can log-in to your cPanel using the link below:-