To view your credits, click Welcome and select Invoices & Orders in the drop-down menu. Credits are marked in red font; the description column indicates the invoice number the credit corresponds to and the Confirmation # will begin with CR.
Click the checkbox next to the invoice you would like to pay as well as the checkbox for the corresponding credit. The credit will be subsequently deducted from the total amount due.
If an unpaid invoice has a credit associated with it, the credit will automatically be selected when you select the invoice. If the credit was issued after the corresponding invoice was paid, then you can manually select the credit to deduct it from an unrelated unpaid invoice.