Once inside the purchase window, click on the orange box that says "View Seat Map".
A new window will appear and you can directly select a seating section from the map on the right or filter the available inventory by number of seats together and/or the price using the table on the left.
If a seating section with two price scales is selected, the available seats will show for both of them.
Click the desired seats and they will automatically be added to your cart.
Click "Review Your Selections" and then proceed to the checkout.