On the client area menu bar, click on the link called My Invoices. This opens a page of all your invoices. Each invoice has a line. Columns include invoice date, invoice due date, invoice total, invoice status (paid, unpaid, overdue) and the last column is View Invoice.
For an unpaid invoice, click on the View Invoice link. You can see the invoice is unpaid. In the upper right corner is a drop-down list of payment methods. Choose your payment method from this menu. Then click the Pay Now button directly below it. After you have completed the payment process, your invoice will be marked as paid. Note that some payment methods will require the invoice to be manually marked as paid.