You can access the payment information for attendees two ways:
attendee/locate the individual/right-click/payment OR
file/payments (under the meeting section) either way you access the same screens
To add a payment....
F8 or the add button
Type in the ID number (the events for that attendee will automatically fill in the left window)
Enter the payment date (space bar for today's date or type in the required date)
Enter the payment amount (these are the minimum, you can complete the rest)
Spread payment if the attendee is paying for all outstanding events.
Pay selected if the attendee is only paying for one of multiple events, you check it in the box.