I put a book in the cart and clicked Proceed to Pay, but the system is taking me to the page of http://pcmbtoday.com/index.php?main_page=login Why am I not getting the payment option?

The reason for this is that you are not logged in. To purchase a book online the system requires your address where books have to be shipped and email id where the order confirmation mail has to be sent. Hence, system prompts the login/registration form that has to be filled in.