Hi Sarah,
If you have credit notes for a particular customer or supplier for whom you also have unpaid bills and invoices, these will simply show up as available within the bills and invoices themselves if you click on the ID numbers. Please be aware that you can only allocate credit notes against unpaid bills and invoices.
If, however, you have some bills and invoices from the same customer supplier, you will have to use a contra account. This is a method of offsetting them against one another. Please pay the invoice (payment to you) into a contra account. Then, use this balance to pay the bill. Any outstanding amount goes into your bank account, where as any shortfall must be met from your account.
I hope that this helps
Chris