Hi,
I have looked through this and can see there is no means to input a Credit note, but I found the following may help you.
You might try raising a new order for the supplier or vendor and put in a negative amount, that is -30.00, for example. In the description put something like "Credit note for Inv 1234". For the 'Item' field you could put "credit note". Then process it as a normal invoice and it should show up as a negative entry that will off-set the amount owing. You should then be able to 'pay' part of the invoice with the 'credit note'. I have given it a test run and it seems to work on the face of it. If you double click the item in the 'Accounts Payable' window you will be able to give it a meaningful ref, say CR3456; and something in the memo field too if you've a mind to.