Click on the linked # to the right of Total Due (for example, if you have two invoices due, it should have [2] and the 2 will be underlined).
This will present a listing of invoices that are due.
The InvNum column has the invoice numbers and they are linked to the detailed invoice which you can also print for your records (for example, invoice 1232 would be underlined and when you click on it, the screen will display that invoice for you).
If you wish to pay by credit card, click the Pay Online button and enter your CC information. If you wish to pay by paypal, click the Paypal button and you'll be taken to Paypal where you can complete the transaction.
As always, if you need help, create a ticket. In this case, please choose Billing for the Department.