Need query to select non-matching fields

[Jen]

Hello. I have two tables that hold account numbers and
monthly payments. The main table has all the monthly
billings that are not paid. The activity table has
monthly billings that have been paid. I want to be able
to select the accounts and the monthly billings that were
input into the activity table, but are not in the main
table. ie... Account 12 has a bill for April in our main
table, they come in and pay a bill for May (that payment
activity populates in the activity table). Now since that
May bill is not in the main table, I want a query to tell
me that. I've tried the 'without matching' query but I
can't seem to get the result. Any help would be
appreciated. Thanks.